MINOR: listener: small API change

MINOR: listener: small API change

A minor API change was performed in listener(.c/.h) to restore consistency
between stop_listener() and (resume/pause)_listener() functions.

LISTENER_LOCK was never locked prior to calling stop_listener():
lli variable hint is thus not useful anymore.

Added PROXY_LOCK locking in (resume/pause)_listener() functions
with related lpx variable hint (prerequisite for #1626).

It should be backported to 2.6, 2.5 and 2.4