doc: Mention zeroed mtimes in the binary tarball.

Fixes <http://bugs.gnu.org/21111>.
Reported by Dave Love <fx@gnu.org>.

* doc/guix.texi (Binary Installation): Add paragraph about "implausibly old
  time stamps" warning from tar.

size: Add '--substitute-urls' option.

* guix/scripts/size.scm (show-help, %options): Add --substitute-urls.
  (%default-options): Add 'substitute-urls'.
  (guix-size): Honor it.
* doc/guix.texi (Invoking guix size): Document it.

doc: Mention initial passwords in "System Installation".

Suggested by remo_ on IRC.

* doc/guix.texi (System Installation): Explicitly mention user account
  passwords.
  (User Accounts): Add 'user-account-password' anchor.

Merge branch 'core-updates'

Merge branch 'master' into core-updates

doc: Update the "Limitations" section.

* doc/guix.texi (GNU Distribution): Use the new package list URL.
  (Limitations): Mention Enlightenment and improve wording.  Update the
  package count.

build-system/ruby: Add #:gem-flags parameter.

* guix/build-system/ruby.scm (build): add 'gem-flags' key
* guix/build/ruby-build-system.scm (build): use 'gem-flags' key
* doc/guix.texi (Build Systems): Mention #:gem-flags.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

file-systems: Add a 'dependencies' field to <file-system>.

* gnu/system/file-systems.scm (<file-system>)[dependencies]: New field.
* gnu/system.scm (other-file-system-services)[requirements]: Honor
  'file-system-dependencies'.
* doc/guix.texi (File Systems): Document it.

system: Add 'kernel-arguments' field.

* gnu/system.scm (<operating-system>)[kernel-arguments]: New field.
  (operating-system-grub.cfg): Honor it.
  (operating-system-parameters-file): Add 'kernel-arguments' to the parameters
  file.
* guix/scripts/system.scm (previous-grub-entries)[system->grub-entry]: Read
  the 'kernel-arguments' field of the parameters file, when available.
* gnu/system/vm.scm (system-qemu-image/shared-store-script):
  Use (operating-system-kernel-arguments os) in '-append'.
* doc/guix.texi (operating-system Reference): Document it.

Add 'guix-daemon.service' file for systemd.

* etc/guix-daemon.service.in: New file, provided by Ricardo Wurmus.
* daemon.am (CLEANFILES): Add etc/guix-daemon.service.
  (systemdservicedir, nodist_systemdservice_DATA): New variables.
  (EXTRA_DIST): Add etc/guix-daemon.service.in.
* doc/guix.texi (Binary Installation, Build Environment Setup): Mention
  'guix-daemon.service'.

doc: Reinstate bit about creating build accounts.

* doc/guix.texi (Binary Installation): Reinstate bit about creating group and
  user accounts, which was inadvertently removed in 01dbc7e0.

Merge branch 'master' into core-updates

file-systems: Allow users to specify file system UUIDs as strings.

Fixes <http://bugs.gnu.org/19778>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/system/file-systems.scm (%uuid-rx): New variable.
  (string->uuid): New procedure.
  (uuid): New macro.
* tests/file-systems.scm: New file.
* Makefile.am (SCM_TESTS): Add it.
* doc/guix.texi (File Systems): Give an example of UUID.

guix build: Add '--substitute-urls' client option.

* guix/scripts/build.scm (%standard-build-options,
  show-build-options-help): Add --substitute-urls=URLS.
  (set-build-options-from-command-line): Honor it.
* guix/store.scm (%default-substitute-urls): Make public.
* doc/guix.texi (Substitutes): Add xref to the client --substitute-urls
  option.
  (Invoking guix build): Document it.
  (Invoking guix-daemon): Add 'daemon-substitute-urls' anchor.

gnu: Require NEON on armhf.

* gnu/packages/gcc.scm (gcc-configure-flags-for-triplet): On armhf,
  include --with-fpu=neon.
* doc/guix.texi (GNU Distribution): Document that our armhf port
  requires NEON.

doc: Augment offloading documentation.

* doc/guix.texi (Daemon Offload Setup): Mention that we're using lsh.  Mention
  GUILE_LOAD_PATH settings on the target machine.  Explain that machines must
  authorize each other.

doc: Remove extra quotes around URLs.

* doc/guix.texi (Invoking guix import): Remove extraneous quotes around URLs.

build: Add 'emacs-build-system'.

* Makefile.am (MODULES): Add 'guix/build-system/emacs.scm' and
  'guix/build/emacs-build-system.scm'.
* guix/build-system/emacs.scm: New file.
* guix/build/emacs-build-system.scm: New file.
* doc/guix.texi (Build Systems): Document it.

import: Add 'elpa' importer.

* guix/import/elpa.scm: New file.
* guix/scripts/import.scm: Add "elpa" to 'importers'.
* guix/scripts/import/elpa.scm: New file.
* Makefile.am (MODULES): Add 'guix/import/elpa.scm' and
  'guix/scripts/import/elpa.scm'.
  (SCM_TESTS): Add 'tests/elpa.scm'.
* doc/guix.texi (Invoking guix import): Document it.
* tests/elpa.scm: New file.
* po/guix/POTFILES.in: Add 'guix/scripts/import/elpa.scm'.

derivations: Add #:substitutable?, distinguished from #:local-build?.

Fixes <http://bugs.gnu.org/18747>.

* guix/derivations.scm (substitutable-derivation?): Rewrite to check for
  "allowSubstitutes".
  (derivation): Add #:substitutable? parameter.
  [user+system-env-vars]: Honor it.
  (build-expression->derivation): Add #:substitutable? and honor it.
* guix/gexp.scm (gexp->derivation): Likewise.
* tests/derivations.scm ("derivation-prerequisites-to-build and substitutes,
  non-substitutable build"): Use #:substitutable? instead of #:local-build?.
  ("substitutable-derivation?", "derivation-prerequisites-to-build and
  substitutes, local build"): New tests.
* guix/download.scm (url-fetch): Adjust comment.
* guix/git-download.scm (git-fetch): Likewise.
* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Use #:substitutable?
  instead of #:local-build?.
* doc/guix.texi (Derivations, G-Expressions): Adjust accordingly.