[FR] Allow specification of local port for caller

When configuring a peer as a caller there appears to be little way to control the local_port setting in the (opaque) peer_config struct.
The rist_create_peer function in udp.c selects an incrementing port value, offset from 32768.

When trying to manually configure the rist_peer_config struct, by forcing the address_family, the physical_port field appears to be used for the destination port. But, AFAICT, the local_port is still not configurable.

In a larger system, with potential races to acquire ports, or a locked down network (e.g. firewall), this can be problematic.