basic: add _cleanup_ wrappers for pthread_mutex_{lock,unlock}

basic: add _cleanup_ wrappers for pthread_mutex_{lock,unlock}

I put the helper functions in a separate header file, because they don't fit
anywhere else. pthread_mutex_{lock,unlock} is used in two places: nss-systemd
and hashmap. I don't indent to convert hashmap to use the helpers, because
there it'd make the code more complicated. Is it worth to create a new header
file even if the only use is in nss-systemd.c? I think yes, because it feels
clean and also I think it's likely that pthread_mutex_{lock,unlock} will be
used in other places later.