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

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'.