Fixes #3507.

Applied the suggested fix from rsc. If the connection
is in closing state then errClosing will bubble up to
the caller.

The fix has been applied to udp, ip and unix as well as
their code path include nil'ing c.fd on close. Func
tests are available in the linked issue that verified
the bug existed there as well.

R=rsc, fullung, alex.brainman, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/6002053