build-system: Add scons-build-system.

* guix/build-system/scons.scm: New file.
* guix/build/scons-build-system.scm: New file.
* Makefile.am (MODULES): Register them.
* doc/guix.texi (Build Systems): Add scons-build-system.

doc: Give the absolute file name of 'hydra.gnu.org.pub' in snippets.

* doc/guix.texi (Substitute Server Authorization): Give the absolute
file name of 'hydra.gnu.org.pub' in the snippet.

gexp: 'compiled-modules' disables deprecation warnings by default.

This avoids repeated deprecation messages, particularly while running
'guix system build' or similar.

* guix/gexp.scm (gexp->derivation): Add #:deprecation-warnings.  Pass it
to 'compiled-modules'.
(compiled-modules): Add #:deprecation-warnings and honor it.
* doc/guix.texi (G-Expressions): Update 'gexp->derivation'
documentation.
* guix/packages.scm (patch-and-repack): Pass #:deprecation-warnings #t.

services: Add Git HTTP(S) service support.

* doc/guix.texi (Version Control Services): Add documentation on the HTTP
backend for git.
* gnu/services/version-control.scm (<git-http-configuration>): New data type.
(git-http-nginx-location-configuration): New helper function.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc: Create "Version Control Services" section.

* doc/guix.texi (Version Control Services): Move to its own node.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

services: Add certbot service.

* gnu/services/certbot.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add new file.
* doc/guix.texi (Certificate Services): New section.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

services: xorg: Remove WindowMaker as a default fallback.

* gnu/services/xorg.scm (xinitrc)[builder](system-profile)
(user-profile): New variables.
(xsession-command, find-session): New procedures.
When FALLBACK-SESSION is #f, find a valid session at run time.
* doc/guix.texi (X Window): Adjust accordingly, and insist that one
or more window managers must be installed.

services: xorg: Document 'slim-configuration' and deprecate 'slim-service'.

* gnu/services/xorg.scm (<slim-configuration>): Provide default values
for all fields.
(slim-service-type)[default-value]: New field.
* doc/guix.texi (X Window): Remove 'slim-service' documentation.
Document 'slim-service-type' and 'slim-configuration'.
* gnu/services/desktop.scm (%desktop-services): Use 'slim-service-type'.

services: xorg: Allow users to specify a list of modules.

* gnu/services/xorg.scm (%default-xorg-fonts): New variable.
(xorg-configuration-file): Add #:modules and #:fonts.  Rewrite to return
a 'computed-file' that honors MODULES and FONTS.
(xorg-wrapper): Pass #:modules to 'xorg-configuration-file'.
(xorg-start-command): Add #:fonts.  Pass #:fonts and #:modules to
'xorg-configuration-file'.
* doc/guix.texi (X Window): Adjust documentation of 'xorg-start-command'
and 'xorg-configuration-file'.

services: configuration: Show default values of list types.

* doc/guix.texi (Messaging Services): Regenerate it.
* gnu/services/configuration.scm (show-default?): Check VAL rather than DEFAULT.
* gnu/services/messaging.scm (show-default?): Check VAL rather than DEFAULT.
(prosody-configuration)[modules-enabled]: Remove default value from docstring.

services: prosody: Add support for disable-sasl-mechanisms.

* doc/guix.texi (Messaging Services): Update accordingly.
* gnu/services/messaging.scm (prosody-configuration)[disable-sasl-mechanisms]:
New field.

services: prosody: Allow to add raw content to the config file.

* doc/guix.texi (Messaging Services): Update accordingly.
* gnu/services/messaging.scm (prosody-configuration)[raw-content]: New field.
(raw-content?, serialize-raw-content): New procedures.

services: prosody: Add support for http-max-content-size.

* doc/guix.texi (Messaging Services): Update accordingly.
* gnu/services/messaging.scm (prosody-configuration)[http-max-content-size]:
New field.

services: prosody: Add support for http-external-url.

* doc/guix.texi (Messaging Services): Update accordingly.
* gnu/services/messaging.scm (prosody-configuration)[http-external-url]: New
field.

services: prosody: Adapt to Prosody 0.10.0.

* doc/guix.texi (Messaging Services): Add "mam" as a module example.  Document
'prosodyctl check'.  Replace 'prosodyctl cert request' with 'prosodyctl cert
import'.  Regenerate it.
* gnu/services/messaging.scm (serialize-module-list): Remove "posix" from the
default modules list because it is now automatically loaded.
(ssl-configuration)[key, certificate]: Remove them because they are now
automatically located.  Fix their docstrings.
(%default-modules-enabled): Add "carbons" and "blocklist".
(prosody-configuration)[certificates]: Set default directory from which
certificates/keys will be automatically located.

doc: Fix typo.

* doc/guix.texi (Bootloader Configuration): Fix ‘choosen’ typo.

doc: Update package count.

* doc/guix.texi (Limitations): Update package count.

doc: Split the "Substitutes" section into subsections.

* doc/guix.texi (Substitutes): Move information into sections, and create one
  new subsection regarding substitution failure.
  (Top): In the detailed listing, add entries for the new subsections.
  (Official Substitute Server): Move info about hydra here.  Add information
  that explains when substitutes are enabled by default.
  (Substitute Server Authorization): Move information about how to enable or
  disable substitutes here.
  (Substitute Authentication): Move information about how Guix authenticates
  substitutes here.
  (Proxy Settings): Move information about how to download substitutes via
  proxy here.
  (Substitution Failure): New section.  Add information about how Guix handles
  substitution failure.  Be extra clear about behavior concerning --fallback.
  (On Trusting Binaries): Move information about trusting binaries here.
  (Invoking guix-daemon): Remove an incorrect statement about when substitutes
  are enabled by default.
  (Common Build Options): Add cross-references for the --fallback option.

Co-authored-by: George Clemmer <myglc2@gmail.com>

Add semicolon in commands that set GUIX_PROFILE.

Fixes <https://bugs.gnu.org/28223>.
Reported by Rosebud Uplink <Uplink.Introversion@outlook.com>.

* doc/guix.texi (Binary Installation): Add missing semicolon after
'GUIX_PROFILE=' line.
(Invoking guix package): Likewise.
* gnu/system.scm (operating-system-etc-service)[profile]: Likewise.
* guix/build/profiles.scm (build-etc/profile): Likewise.

doc: Write "file system" instead of "partition" as appopriate.

* doc/guix.texi (Preparing for Installation): Write "file system" instead
of "partition" as appopriate.