MEDIUM: init: stop disabled proxies after initializing fdtab

MEDIUM: init: stop disabled proxies after initializing fdtab

During the startup process we don't have any fdtab nor fd_updt for quite
a long time, and as such some operations on the listeners are not
permitted, such as fd_want_*/fd_stop_* or fd_delete(). The latter is of
particular concern because it's used when stopping a disabled frontend,
and it's performed very early during check_config_validity() while there
is no fdtab yet. The trick till now relies on the listener's state which
is a bit brittle.

There is absolutely no valid reason for stopping a proxy's listeners this
early, we can postpone it after init_pollers() which will at least have
allocated fdtab.