BUG/MEDIUM: listener: don't report an error when resuming unbound listeners

BUG/MEDIUM: listener: don't report an error when resuming unbound listeners

Pavlos Parissis reported that a sequence of disable/enable on a frontend
performed on the CLI can result in an error if the frontend has several
"bind" lines each bound to different processes. This is because the
resume_listener() function returns a failure for frontends not part of
the current process instead of returning a success to pretend there was
no failure.

This fix should be backported to 1.5.