system: Make /gnu/store a read-only bind mount by default.

* gnu/system/file-systems.scm (%immutable-store): New variable.
  (%base-file-systems): Add it.
* doc/guix.texi (File Systems): Document it.

guix package: -A and -s take supported systems into account.

* guix/scripts/package.scm (guix-package)[process-query]
  <list-available>: Restrict results to packages matching
  'supported-package?".
* guix/ui.scm (package->recutils): Print "systems:".
* tests/guix-package.sh: Add tests.
* doc/guix.texi (Invoking guix package): Adjust description of
  '--list-available' accordingly.

doc: Warn earlier about the risk of overwriting with the binary install.

Suggested by Pjotr Prins <pjotr.public12@thebird.nl>.

* doc/guix.texi (Binary Installation): Move warning about overwriting
  higher up.

packages: Allow package lookups with version prefixes.

* gnu/packages.scm (find-packages-by-name): Sort MATCHING according to
  'version>?'.  Use 'string-prefix?' instead of 'string=?' to compare
  against VERSION.
* doc/guix.texi (Invoking guix package): Add example and explanation.

Merge branch 'core-updates'

Merge branch 'master' into core-updates

Conflicts:
	gnu-system.am
	gnu/packages/gstreamer.scm

refresh: Allow users to refer to specific package versions.

* guix/scripts/refresh.scm (guix-refresh): Use 'specification->package'
  instead of 'find-packages-by-name'.  This allows users to specify
  things like "qt-4.8.6".
* doc/guix.texi (Invoking guix refresh): Add an example.

doc: Add section on installation from a binary tarball.

* doc/guix.texi (Binary Installation): New section.

guix package: Add '--do-not-upgrade' option.

* guix/scripts/package.scm (%options): Add the '--do-not-upgrade' option.
  (show-help): Document it.
  (options->installable): Add 'do-not-upgrade-regexps' variable.
  Use it in 'packages-to-upgrade'.
* doc/guix.texi (Invoking guix package): Document the '--do-not-upgrade'
  option.

Merge branch 'master' into core-updates

services: Add upower.

* gnu-system.am (GNU_SYSTEM_MODULES): Add gnu/services/upower.scm.
* gnu/services/upower.scm: New file, defining a upower service.
* doc/guix.texi (Various Services): Mention it.

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

doc: Add example showing how to customize 'nscd-service'.

* doc/guix.texi (Name Service Switch): Add link to nss-mdns.  Add
  example show how to pass #:name-services to 'nscd-service'.
  (Base Services): Add link from 'nscd-service' to the above.
* gnu/services/base.scm (nscd-service): Update docstring accordingly.

Merge branch 'master' into core-updates

activation: Remove undeclared user accounts and groups.

Fixes <http://bugs.gnu.org/19795>.
Reported by David Thompson <dthompson2@worcester.edu>.

* gnu/build/activation.scm (enumerate, current-users, current-groups,
  delete-user, delete-group): New procedures.
  (activate-users+groups): Add calls to 'delete-user' and
  'delete-group'.
* doc/guix.texi (User Accounts): Add a paragraph about statelessness.
  Explain that passwords are preserved.

import: Add hackage importer.

* guix/scripts/import.scm (importers): Add hackage.
* guix/scripts/import/hackage.scm: New file.
* po/guix/POTFILES.in: Add guix/scripts/import.scm.
* doc/guix.texi: Add section on 'hackage' importer.

gnu: Add colord service.

* gnu-system.am (GNU_SYSTEM_MODULES): Add gnu/services/colord.scm.
* gnu/services/colord.scm: New file.
* doc/guix.texi (Various Services): Document it.

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

Merge branch 'master' into core-updates

doc: Refer to "Substitutes" from "Setting Up the Daemon".

Suggested by Mark H Weaver.

* doc/guix.texi (Setting Up the Daemon): Add xref to "Substitutes".

doc: Mention the relation between module names and file names.

* doc/guix.texi (Package Modules): Mention the relation between module
  names and file names.

doc: Mention that the current generation is not deleted.

* doc/guix.texi (Invoking guix package): Mention that the current
  generation is not deleted.  This is a followup to d26eb84.