Rx: Make UDP ports 16-bit to compile (GH #868).

Rx: Make UDP ports 16-bit to compile (GH #868).

Gisle Vanem reported that GCC 7.1 for DJGPP sees u_int and uint32_t as
two different types, hence the forward declaration for rx_cache_find()
was different from the actual function. UDP port numbers are 16-bit and
udp_print() appropriately uses uint16_t for them. Use the same type in
the downstream Rx-specific code to make things simpler and consistent.