MEDIUM: proxy: zombify proxies only when the expose-fd socket is bound

MEDIUM: proxy: zombify proxies only when the expose-fd socket is bound

When HAProxy is running with multiple processes and some listeners
arebound to processes, the unused sockets were not closed in the other
processes. The aim was to be able to send those listening sockets using
the -x option.

However to ensure the previous behavior which was to close those
sockets, we provided the "no-unused-socket" global option.

This patch changes this behavior, it will close unused sockets which are
not in the same process as an expose-fd socket, making the
"no-unused-socket" option useless.

The "no-unused-socket" option was removed in this patch.