services: Make 'nscd-service' configurable; cache hosts/services by default.

Before that, as it was given an empty configuration file, nscd would
actually have all its caches disabled.

* gnu/services/base.scm (<nscd-configuration>, <nscd-cache>): New record
  types.
  (%nscd-default-caches, %nscd-default-configuration): New variables.
  (nscd.conf-file): New procedure.
  (nscd-service): Add 'config' parameter.  Use 'nscd.conf-file', and
  pass its result as the '-f' parameter of nscd.
* doc/guix.texi (Base Services): Update 'nscd-service' documentation
  accordingly.  Document 'nscd-configuration', 'nscd-cache',
  '%nscd-default-configuration', and '%nscd-default-caches'.

services: user-unmount: Change directory to "/" before unmounting.

* gnu/services/base.scm (user-unmount-service): Add 'chdir' call.

services: Add 'user-unmount-service' as an essential service.

* gnu/services/base.scm (user-unmount-service): New procedure.
* gnu/system.scm (essential-services): Use it.
* gnu/system/install.scm (cow-store-service): Mention it in comment.

services: udev: Make /dev/kvm 666 as a workaround.

* gnu/services/base.scm (kvm-udev-rule): Change /dev/kvm mode to 666 to
  work around <http://bugs.gnu.org/18994>.

services: Increase delay before SIGKILL is sent when halting.

* gnu/services/base.scm (user-processes-service): Increase the default
  value of 'grace-delay'.  This leaves more time for processes to clean
  up their PID files, /tmp/.X0-lock, etc.

system: Fix value of 'EUDEV_RULES_DIRECTORY'.

* gnu/services/base.scm (udev-service): Append "/lib/udev/rules.d" to
  EUDEV_RULES_DIRECTORY.

system: Add udev rule for /dev/kvm.

* gnu/services/base.scm (kvm-udev-rule): New procedure.
  (udev-service): Use it, and add it to RULES.
  (guix-build-accounts): Add "kvm" to the 'supplementary-groups' field.
* gnu/system/shadow.scm (%base-groups): Add "kvm".

system: Add support for swap devices.

* gnu/services/base.scm (swap-service): New procedure.
* gnu/system.scm (<operating-system>)[swap-devices]: New field.
  (swap-services): New procedure.
  (essential-services): Use it.

system: Define 'device-mapping-kind', and add a 'close' procedure.

* gnu/system/file-systems.scm (<mapped-device-type>): New record type.
  (<mapped-device>)[command]: Remove field.
  [type]: New field.
* gnu/services/base.scm (device-mapping-service): Rename 'command'
  parameter to 'open'.  Add 'close' parameter and honor it.
* gnu/system.scm (luks-device-mapping): Rename to...
  (open-luks-device): ... this.
  (close-luks-device): New procedure.
  (luks-device-mapping): New variable.
  (device-mapping-services): Get the type of MD, and pass its 'open' and
  'close' fields to 'device-mapping-service'.

services: file-system: Change directory to root before unmounting.

* gnu/services/base.scm (file-system-service)[stop]: Add 'chdir' call.

services: user-processes: Wait for complete process termination.

* gnu/services/base.scm (user-processes-service): Add 'wait' loop.

services: udev: Make non-respawnable.

* gnu/services/base.scm (udev-service): Add 'respawn?' field.

Merge branch 'master' into core-updates

Conflicts:
	gnu-system.am

system: Use eudev instead of udev by default.

* gnu/system.scm (%base-packages): Change 'udev' to 'eudev'.
* gnu/services/base.scm (udev-service): Change #:udev to default to
  'eudev'.

services: udev: Allow the use of eudev.

* gnu/services/base.scm (udev-service): Adjust 'start' method to support
  eudev in addition to udev.

system: Add FUSE and ALSA udev rules by default.

* gnu/services/base.scm (%base-services): Pass FUSE and ALSA-UTILS to
  'udev-service'.

Merge branch 'master' into core-updates

Conflicts:
	gnu/packages/image.scm

system: Add support for Linux-style mapped devices.

* gnu/system/file-systems.scm (<mapped-device>): New record type.
* gnu/system.scm (<operating-system>)[mapped-devices]: New field.
  (luks-device-mapping): New procedure.
  (other-file-system-services)[device-mappings, requirements]: New
  procedures.  Pass #:requirements to 'file-system-service'.
  (device-mapping-services): New procedure.
  (essential-services): Use it.  Append its result to the return value.
  (operating-system-initrd-file): Add comment.
* gnu/services/base.scm (file-system-service): Add #:requirements
  parameter and honor it.
  (device-mapping-service): New procedure.
* gnu/system/linux-initrd.scm (base-initrd): Add comment.

services: udev: Allow for additional udev rules.

* gnu/services/base.scm (udev-rules-union): New procedure.
  (udev-service): Add #:rules parameter.  Call 'udev-rules-union' and
  create udev.conf.  Set $UDEV_CONFIG_FILE before spawning udevd.

Merge branch 'master' into core-updates