MINOR: listener: allow thread kw for rhttp bind

MINOR: listener: allow thread kw for rhttp bind

Thanks to previous commit, a reverse HTTP listener is able to distribute
actively opened connections accross its threads. To be able to exploit
this, allow "thread" keyword for such a listener.

An extra check is added to explicitely forbids a reverse bind to span
multiple thread groups. Without this, multiple listeners instances will
be created, each with its owned "nbconn" value. This may surprise users
so for now, better to deactivate this possibility.