Make udp_physical_addr() and udp_protocol_addr() static.

They aren't used outside udp.c

Signed-off-by: Stephan Gatzka <stephan.gatzka@gmail.com>
Make functions and structs dealing with string literals const correct.

This allows the compiler to check if someone writes into string
literals.

Signed-off-by: Stephan Gatzka <stephan.gatzka@gmail.com>