Support logging to syslog

Simply set the logfile path to "syslog" and murmur will log to syslog.

This should be stable, it has approximately 4 user-months of testing.
Thanks for the clarification.  My man page on Ubuntu does not make it clear that a `0` facility means "use the default: LOG_USER", which may confuse people reading this in the future.

Taking that into account, I think we should use `LOG_USER` instead of `0` for the sake of maintainability.