racket/network: improve UDP support

Generalize `udp-send-to`, etc., to try each possibility of
a resolved address (instead of just the first one) like
`udp-connect!` does. This matters, for example, when using
"localhost" as an address, when the machine resolves "locahost"
to both "127.0.0.1" and "::1", and when the socket is created
for the second one that would be tried.

Also, detect and discard asynchronous ICMP errors.
racket/gui Cocoa: fix test for 10.9