Fix typos in manual.

* doc/guix.texi (Using the Configuration System): 'gid' -> 'group'
  in user-account example.
  (Initial RAM Disk): 'base-init' -> 'base-initrd'.

doc: Add "Setuid Programs" node.

* doc/guix.texi (Setuid Programs): New node.

doc: Add "Initial RAM Disk" section.

* doc/guix.texi (Initial RAM Disk): New node.
* gnu/system/linux-initrd.scm (expression->initrd): Adjust docstring.

doc: List noteworthy limitations.

* doc/guix.texi (Limitations): New subsection.

doc: Move "System Configuration" higher.

* doc/guix.texi (GNU Distribution): Move "System Configuration" right
  after "System Installation".

doc: Add a "System Installation" node.

* doc/guix.texi (Installation): Add cross-ref to "System Installation".
  (System Installation): New section.
  (System Configuration): Remove disclaimer.

doc: Remove redundant listing of services.

* doc/guix.texi (Defining Services): Remove listing of service
  procedures; add xref to "Services".

services: Add options to 'guix-service'.

* gnu/services/base.scm (guix-service): Add #:use-substitutes? and
  #:extra-options parameters, and honor them.
* doc/guix.texi (Base Services): Adjust accordingly.

services: Add Tor service.

* gnu/services/networking.scm (tor-service): New procedure.
* doc/guix.texi (Networking Services): Document it.
* build-aux/hydra/demo-os.scm: Use it.  Add TOR and TORSOCKS to
  'packages'.

doc: Start writing about services.

* doc/guix.texi (Using the Configuration System): Remove details about
  '%base-services', and link to "Services".
  (Services): Add introductory text.
  (Base Services, Networking Services, X Window): New nodes.

guix system: Add '--system' option.

* guix/scripts/system.scm (switch-to-system): Add #:system parameter;
  pass it to 'run-with-store'.
  (%options): Add '--system'.
  (guix-system): Pass the 'system' option to 'run-with-store',
  'package-derivation', and 'switch-to-system' calls.
* doc/guix.texi (Invoking guix system): Document '--system' and
  '--image-size'.

doc: Document user accounts and user groups.

* doc/guix.texi (File Systems): Fix typo.
  (User Accounts): Populate.

doc: Write about file system configuration.

* doc/guix.texi (Using the Configuration System): Change 'guix system
  boot' to 'guix system reconfigure'.
  (File Systems, User Accounts, Services): New nodes.

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".