services: nscd: Add 'invalidate' and 'statistics' actions.

* gnu/services/base.scm (nscd-action-procedure, nscd-actions): New
procedures.
(nscd-shepherd-service): Add 'modules' and 'actions' fields.
* gnu/tests/base.scm (run-basic-test)["nscd invalidate action"]
["nscd invalidate action, wrong table"]: New tests.
* doc/guix.texi (Services): Mention 'herd doc nscd action'.
(Base Services): Document the actions.