MEDIUM: proto_ux: properly suspend named UNIX listeners

MEDIUM: proto_ux: properly suspend named UNIX listeners

When a listener is suspended, we expect that it may not process any data for
the time it is suspended.

Yet for named UNIX socket, as the suspend operation is a no-op at the proto
level, recv events on the socket may still be processed by the polling loop.

This is quite disturbing as someone may rely on a paused proxy being harmless,
which is true for all protos except for named UNIX sockets.

To fix this behavior, we explicitely disable io recv events when suspending a
named UNIX socket listener (we call disable() method on the listener).

The io recv events will automatically be restored when the listener is resumed
since the l->enable() method is called at the end of the resume() operation.

This could be backported up to 2.4 after a reasonable observation
period to make sure that this change doesn't cause unwanted side-effects.