BUG/MEDIUM: listener: fix pause_listener() suspend return value handling

BUG/MEDIUM: listener: fix pause_listener() suspend return value handling

Legacy suspend() return value handling in pause_listener() has been altered
over the time.

First with fb76bd5ca ("BUG/MEDIUM: listeners: correctly report pause() errors")
Then with e03204c8e ("MEDIUM: listeners: implement protocol level
->suspend/resume() calls")

We aim to restore original function behavior and comply with resume_listener()
function description.
This is required for resume_listener() and pause_listener() to work as a whole

Now, it is made explicit that pause_listener() may stop a listener if the
listener doesn't support the LI_PAUSED state (depending on the protocol
family, ie: ABNS sockets), in this case l->state will be set to LI_ASSIGNED
and this won't be considered as an error.

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.

--
Backport notes:

This commit depends on:
 - "MINOR: listener: make sure we don't pause/resume bypassed listeners"

-> 2.4: manual change required because "MINOR: proxy/listener: support
for additional PAUSED state" was not backported: the contextual patch
lines don't match.

Replace this:

    |        if (px && !px->li_ready) {
    |                /* PROXY_LOCK is required */

By this:

    |        if (px && !px->li_ready) {
    |               ha_warning("Paused %s %s.\n", proxy_cap_str(px->cap), px->id);