We are going to add more UDP services, so this commit adds the
generic management of UDP sockets, with udp protocol and port matching.
Thus, we now have two IPV4 sockets: one for ICMP and one for bootp;
whose receive part relies on generic code for the match.

We still miss the output helpers, so with this commit bootp still
builds the whole frame by itself.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>