BUG/MEDIUM: unix: completely unbind abstract sockets during a pause()

Abstract namespace sockets ignore the shutdown() call and do not make
it possible to temporarily stop listening. The issue it causes is that
during a soft reload, the new process cannot bind, complaining that the
address is already in use.

This change registers a new pause() function for unix sockets and
completely unbinds the abstract ones since it's possible to rebind
them later. It requires the two previous patches as well as preceeding
fixes.

This fix should be backported into 1.5 since the issue apperas there.
(cherry picked from commit fd0e008d9d4db2f860b739bd28f6cd31d9aaf2b5)
diff --git a/include/proto/proto_uxst.h b/include/proto/proto_uxst.h
index 9422ea7..8e796ec 100644
--- a/include/proto/proto_uxst.h
+++ b/include/proto/proto_uxst.h
@@ -27,6 +27,7 @@
 #include <types/task.h>
 
 void uxst_add_listener(struct listener *listener);
+int uxst_pause_listener(struct listener *l);
 int uxst_get_src(int fd, struct sockaddr *sa, socklen_t salen, int dir);
 int uxst_get_dst(int fd, struct sockaddr *sa, socklen_t salen, int dir);