doc: Improve wording and example in "Using the Configuration System".

* doc/guix.texi (Using the Configuration System): Make the intro more
  user-oriented.  Change example to return an operating system.  Fix
  typos.  Mention mandatory/optional fields.

guix system: Add 'reconfigure' action.

* guix/scripts/system.scm (%system-profile): New variable.
  (switch-to-system, previous-grub-entries): New procedures.
  (unless-file-not-found): New macro.
  (show-help): Add 'reconfigure'.
  (guix-system): Handle it.
* gnu/system.scm: Export 'operating-system-activation-script'.
* doc/guix.texi (Invoking guix system): Document it.

Merge branch 'core-updates'

doc: Mention KVM support for 'guix system'.

* doc/guix.texi (Invoking guix system): Add note about KVM support.

Merge branch 'master' into core-updates

guix build: Allow gexps to be passed to '-e'.

* guix/ui.scm (%guix-user-module): New variable.
  (read/eval): Pass it as the second argument to 'eval'.
* guix/scripts/build.scm (options/resolve-packages): Add case for
  'gexp?'.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix build): Document '-e gexp'.
guxi build: Allow gexps to be passed to '-e'.

* guix/ui.scm (%guix-user-module): New variable.
  (read/eval): Pass it as the second argument to 'eval'.
* guix/scripts/build.scm (options/resolve-packages): Add case for
  'gexp?'.
* tests/guix-build.sh: Add tests.
* doc/guix.texi (Invoking guix build): Document '-e gexp'.

doc: Add a few words on the term "derivation".

* doc/guix.texi (Programming Interface): Add a sentence on "derivation".

Merge branch 'master' into core-updates

derivations: 'build-expression->derivation' supports #:allowed-references.

* guix/derivations.scm (build-expression->derivation): Add
  #:allowed-references and pass it to 'derivation.
* doc/guix.texi (Derivations): Adjust accordingly.

system: Define '%base-packages' and use it.

* gnu/system.scm (<operating-system>)[packages]: Change default value to
  %BASE-PACKAGES.
  (%base-packages): New variable.
* gnu/system/install.scm (installation-os): Use it when defining the
  'packages' field.
* doc/guix.texi (Using the Configuration System): Use %BASE-PACKAGES in
  example.  Remove now unneeded module imports.  Explain this.

derivations: Add #:allowed-references 'derivation' parameter.

* guix/derivations.scm (derivation): Add #:allowed-references
  parameter.
  [user+system-env-vars]: Honor it.
* tests/derivations.scm ("derivation #:allowed-references, ok",
  "derivation #:allowed-references, not allowed",
  "derivation #:allowed-references, self allowed",
  "derivation #:allowed-references, self not allowed"): New tests.
* doc/guix.texi (Derivations): Document #:allowed-references.

linux-initrd: Allow use of volume labels in 'file-system' declarations.

* guix/build/linux-initrd.scm (%ext2-endianness, %ext2-sblock-magic,
  %ext2-sblock-creator-os, %ext2-sblock-uuid, %ext2-sblock-volume-name):
  New macros.
  (read-ext2-superblock, ext2-superblock-uuid,
  ext2-superblock-volume-name, disk-partitions,
  partition-label-predicate, find-partition-by-label,
  canonicalize-device-spec): New procedures.
  (mount-file-system): Use 'canonicalize-device-spec' on SOURCE.
  (boot-system): Likewise for ROOT.
* doc/guix.texi (Using the Configuration System): Adjust 'file-system'
  declaration accordingly.

Merge branch 'master' into core-updates

doc: Augment "Package Naming".

* doc/guix.texi (Package Naming): Mention underscores, and give SDL_net
  as an example.

services: nscd: Provide an 'activate' script to make /var/run/nscd.

* gnu/services/base.scm (nscd-service): Add 'activate' field.
* guix/build/install.scm (directives): Remove /var/run/nscd; add
  /var/run.
* doc/guix.texi (Defining Services): Add 'activate' field in example.
  Document it.

guix system: Add 'disk-image' action.

* guix/scripts/system.scm (show-help): Add 'disk-image'.
  (guix-system)[parse-options]: Support 'disk-image' action.
  [option-arguments]: Likewise.
  Handle the 'disk-image' action.
* doc/guix.texi (Invoking guix system): Document 'disk-image'.

Add (gnu) module.

* gnu.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
* build-aux/hydra/demo-os.scm: Use (gnu) and strip import list
  accordingly.
* doc/guix.texi (Using the Configuration System): Adjust example
  accordingly.

doc: Show the 'bootloader' field in system example.

* doc/guix.texi (Using the Configuration System): Add 'bootloader'
  field.

guix system: 'guix system init' installs GRUB by default.

* guix/scripts/system.scm (install): Add #:grub?, #:grub.cfg, and
  #:device parameters; honor them.
  (show-help): Document '--no-grub'.
  (%options): Add '--no-grub'.
  (%default-options): Add 'install-grub?'.
  (guix-system): Honor 'install-grub?' option from OPTS.  Adjust
  'install' call accordingly.
* doc/guix.texi (Invoking guix system): Document '--no-grub'.

guix system: Add 'init' sub-command.

* guix/scripts/system.scm (install): New procedure.
  (guix-system)[parse-option]: Remove check for extraneous arguments.
  [match-pair, option-arguments]: New procedures.
  Use 'option-arguments'.  Honor 'init'.
  (show-help): Document 'init'.
* doc/guix.texi (Invoking guix system): Document 'init'.