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 looks good, although I don't think the fact nscd is not running/not found are INFO conditions (which I think are printed during every run of ncm-ncd) but more like debug level stuff.