DEV: udp: add a tiny UDP proxy for testing

DEV: udp: add a tiny UDP proxy for testing

QUIC really needs more traffic perturbation for the tests. Let's have
a tiny UDP proxy for this, mostly derived from the 'connect' test suite.
For now it only supports a single "connection" at once, but the code is
here to support more. A new "connection" will simply replace the previous
one. It doesn't yet cause traffic perturbations, this is still to be added.
Some of the setsockopt() are possibly unneeded. The error handling is
almost inexistent, and polling for sends is not implemented at all (will
cause losses). No stats are collected.