Add naive method to invalidate nscd caches when databases change

TODO: Check whether system has nscd and whether it is running.
Also unit tests.

Fixes #47 but is currently rubbish, however this might annoy someone enough that they improve my code.
this can possibly find `nscd` running with other binary then in `/usr/sbin/nscd`. not sure if it is an issue. and it requires `pgrep` to be installed, so add it as dependency (and not via rpm name)