lib-master: Fix calling avail_overflow_callback with service_count=1

lib-master: Fix calling avail_overflow_callback with service_count=1

This is used by login processes to kill the oldest connection when all of
the clients and processes for the service are taken. This wasn't working
when service_count=1 because the listeners were closed already.

It's not really useful anymore to even do this kind of a listener closing.
It was originally added in 9217d2426b4f8ece47441357f35d8bb34d97c4eb to
prevent restart problems with long-running login processes. However,
nowadays listeners can be closed via SIGQUIT so this isn't necessary.