Allow invalidating the nfsidmap cache

This introduces an nfsidmap value for nscd_invalidate which will cause
the nfsidmap -c command to be run.
Rename nscd_invalidate option to reconnect_invalidate

This also renames the internal nscd module to invalidator for both nslcd
and pynslcd. The new invalidator module is now no longer nscd-specific.
Make cache invalidation more generic

This changes the nscd_invalidate option into a more generic
reconnect_invalidate and also allows clearing the nfsidmap cache.