doc: Merge the type, variable, and function indices.

* doc/guix.texi (Function Index): Rename to...
  (Programming Index): ... this.  Merge the type, variable, and function
  indices.

system: Add 'file-system' decl. for /dev/pts, and use the right options.

Fixes <http://bugs.gnu.org/18081>.

* gnu/system/file-systems.scm (%devtmpfs-file-system): Add
  'needed-for-boot?' field.
  (%tty-gid, %pseudo-terminal-file-system): New variables.
  (%base-file-systems): Add %PSEUDO-TERMINAL-FILE-SYSTEM.
* gnu/services/base.scm (udev-service): Remove dependency on
  'file-system-/dev'.
* gnu/system/shadow.scm (%base-groups): Add 'id' field for group 'tty'.
* guix/build/linux-initrd.scm (boot-system): Remove 'mount' call for
  /dev/pts.
* doc/guix.texi (File Systems): Add %pseudo-terminal-file-system.

system: Add 'create-mount-point?' file system option.

* gnu/system/file-systems.scm (<file-system>)[create-mount-point?]: New
  field.
* gnu/services/base.scm (file-system-service): Add #:create-mount-point?
  parameter and honor it.
* gnu/system.scm (other-file-system-services): Update
  'file-system-service' call accordingly.
* doc/guix.texi (File Systems): Document it.

system: Add '%devtmpfs-file-system' for udev, and '%base-file-systems'.

Suggested by Adam Pribyl <pribyl@lowlevel.cz>.

* gnu/services/base.scm (udev-service)[requirement]: Add
  'file-system-/dev'.
* gnu/system/file-systems.scm (%devtmpfs-file-system,
  %base-file-systems): New variables.
* gnu/system/install.scm (installation-services)[file-systems]: Use
  %base-file-systems.
* build-aux/hydra/demo-os.scm (file-systems): Likewise.
* doc/guix.texi (System Installation): Show %BASE-FILE-SYSTEMS in the
  example.
  (Using the Configuration System): Likewise.
  (File Systems): Document %base-file-systems, %devtmpfs-file-system,
  %binary-format-file-system, and %fuse-control-file-system.

doc: Fix typo.

* doc/guix.texi (System Installation): s/are/is/.

guix package: add a "show" option.

* doc/guix.texi: Update the documentation.
* guix/scripts/package.scm: Add a "show" option.
* tests/guix-package.sh: Add a test for the "show" option.

doc: Show output of 'guix refresh --list-dependent' example.

* doc/guix.texi (Invoking guix refresh): Show example output of the
  command.

guix: refresh: Add --list-dependent option.

* guix/packages.scm (package-direct-inputs): New procedure.
* gnu/packages.scm (vhash-refq, package-direct-dependents)
  (package-transitive-dependents, package-covering-dependents): New procedures.
* guix/scripts/refresh.scm (%options, show-help, guix-refresh): Add
  --list-dependent option.
* doc/guix.texi (Invoking guix refresh): Document '--list-dependent' option.

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.