rrdcached enhancements to drop unused privileges

Init systems generally support invoking daemons with reduced privileges, but because rrdcached manages its own pid file and UNIX domain sockets in restricted directories those features can't be used.  This series of commits ultimately provides flags to rrdcached that specify a group and user that the daemon should run as after all activities that might require higher privileges have completed successfully.  On the way there, a couple cleanups are provided.