reference: simplify some contractcs
racket/udp: fixed for `udp-bind!' and `udp-connect!'

Fix ephemeral-port support in `udp-bind!', and
change `udp-bind!' and `udp-connect!' to try address
resolutions in order to find one that works (which is
typically needed to auto-select an IPv4 or IPv6 variant
of an address).
fix a UDP multicast test to make it work on Windows XP