BUG/MEDIUM: fix broken send_proxy on FreeBSD

David Berard reported that send-proxy was broken on FreeBSD and tracked the
issue to be an error returned by send(). We already had the same issue in
the past in another area which was addressed by the following commit :

   0ea0cf6 BUG: raw_sock: also consider ENOTCONN in addition to EAGAIN

In fact, on Linux send() returns EAGAIN when the connection is not yet
established while other OSes return ENOTCONN. Let's consider ENOTCONN for
send-proxy there as the same as EAGAIN.

David confirmed that this change properly fixed the issue.

Another place was affected as well (health checks with send-proxy), and
was fixed.

This fix does not need any backport since it only affects 1.5.