net: add support for UDP sockets, use it (receive only by now)

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>