s3-prefork: Fix code to retire children

s3-prefork: Fix code to retire children

We have to be more careful when retiring children. We cannot stop
accepting connections as soon as the server tells us to quit because if
max_children is reached and we still have clients connected, the server will
not be able to spawn new children until one exits. And children will not exit
until all the clients closed the connections. So we keep operating past our
recall until we have 0 connections.
Also do not try to recall children that still have >= 1 clients connected, they
couldn't anyway.
Also use messaging to warn children and not SIGHUP.

Signed-off-by: Andreas Schneider <asn@samba.org>
Signed-off-by: Simo Sorce <idra@samba.org>