[MultiSync] Don't set source address, let the OS figure it out

This should fix the issue of using MultiSync over wireless when
E1.31 is going out eth0.  Previously the MultiSync code was
temporarily using the E131Interface setting to decide which
interface to use for sync packets.
[MultiSync] UI fixes for MultiSync

Clean up the logs and warnings a little to make them clearer.
[GPIO] Add a way to enable the PiFace GPIO config section.

This is manual for now, in the near future, the PiFace should be
automatically detected at boot time so the screen can be
properly generated without user intervention.