Bind audio stream, timing and control sockets to available port

Hi Mike,

Thank you very much for all your great work on shairport-sync. The synchronisation is a great feature to have on top of the excellent work by @abrasive.

I am running multiple shairport instances on a machine with multiple audio interfaces. This worked great with 
shairport, however with shairport-sync I get an `could not bind a UDP port!` error.

Herewith a small change to `rtp.c` that binds audio stream, timing and control sockets to available port instead of hard coded ports 6000, 6001, 6002. This seems to do the trick for me.

Pieter