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.

services: guix: Remove dependency on 'lsof'.

This is a followup to b8f59cdc20e9d83ce63523ef917e95fcee07f134:
'list-runtime-roots' no longer depends on 'lsof'.

* gnu/services/base.scm (<guix-configuration>)[lsof]: Remove.
(guix-shepherd-service): Adjust accordingly.
* doc/guix.texi (Base Services): Adjust accordingly.
* gnu/system.scm (%base-packages): Remove LSOF.

doc: Better explain GC roots.

Fixes <https://bugs.gnu.org/24194>.
Reported by J. Eppler <j.eppler@openmailbox.org>.

* doc/guix.texi (Invoking guix-daemon): Add index entries for GC roots.
Add xref to "Invoking guix gc".
(Invoking guix gc): Add the string "GC roots"; mention /var/guix/gcroots.
(Additional Build Options): Add index entries.  Better explain --root.
(Invoking guix environment): Add xref to "Invoking guix gc".

doc: Move paragraph about signature verification to the top.

* doc/contributing.texi (Submitting Patches): Remind contributors to verify
cryptographic signatures at the very beginning.

services: Add murmur.

* gnu/services/telephony.scm: New file.
* gnu/local.mk: Add it.
* doc/guix.texi (Telephony Services): New node.

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

doc: Fix typo in documentation of 'guix graph'.

* doc/guix.texi (Invoking guix graph): Fix typo.

doc: Document the installation of Go source code.

* doc/guix.texi (Build Systems): Document '#:install-source?' for the Go
build system.

challenge: Display an overall summary.

* guix/scripts/challenge.scm (summarize-report-list): New procedure.
(guix-challenge): Use it.
* doc/guix.texi (Invoking guix challenge): Adjust command output in
example.

gexp: Add 'directory-union'.

* gnu/services.scm (directory-union): Move to...
* guix/gexp.scm (directory-union): ... here.  New procedure.
* doc/guix.texi (G-Expressions): Document it.

gexp: Add 'file-union'.

* gnu/services.scm (file-union): Move to...
* guix/gexp.scm (file-union): ... here.  New procedure.
* doc/guix.texi (G-Expressions): Document it.

services: base: Add file->udev-rule function.

This function allows passing a file-like object to the udev service.

* gnu/services/base.scm (file->udev-rule): New function.
* doc/guix.texi (Base Services): Document it.

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