util: unify SO_PEERCRED/SO_PEERSEC invocations

Introduce new call getpeercred() which internally just uses SO_PEERCRED
but checks if the returned data is actually useful due to namespace
quirks.
bus: only accept kdbus creds if they are valid

This allows userspace to fake kdbus creds via struct ucred in the proxy,
without making the recieving side choke on the missing fields of the
kdbus struct, more precisel pid_starttime and tid
util: don't accept an empty peer label as valid