Avoid unhelpful warnings about UDP buffer sizes.

Don't log a warning if we never explicitly configured the
SO_RCVBUF/SO_SNDBUF sizes, and don't warn if the system allocates a
larger buffer than the one requested, as at least on Linux, it will
always double the requested size unless you hit the maximum. With this
change, we only warn when we explicitly request a buffer size and the
system allocated a smaller one.