Default to port 5060 when responding over UDP where the port is unspecified

This occurs when the Via header is not receiver tagged, and thus
follows Section 5 of RFC 3263 where the default port for the protocol
should be utilise if not specified in the sent-by.

This remains unnoticed by the tests due to the NkSIP UAC always
including the port in the 'Via' sent-by field.

I've tried to keep this change somewhat low-impact with the new
engine drop in mind.  This could probably do with revisiting to normalise
this decision path for selecting a transport.