ncm-accounts: 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.
not to delay this even more, but can you rename this `invalidate_nscd_cache` and make sure all messages logged report it as such (`nscd cache` instead of just `cache`)? `sssd` also has caches, maybe at some point we'll want them to be cleared for the same reasons...