smbd: Implement a cleanup daemon

smbd: Implement a cleanup daemon

We do way too much stuff in the parent smbd in remove_child_pid(). In
particular accessing ctdbd is not a good idea when ctdbd is stuck in something.
We've had a case where smbd exited itself with "ctdb timeout" being set to 60
seconds. ctdb was just stuck doing recoveries, and the parent smbd was sitting
in serverid_exists trying to retrieve a record for a child that had exited. Not
good.

This daemon sits there as parent->cleanupd and receives MSG_SMB_NOTIFY_CLEANUP
messages that hold the serverid and exit status of a former child. The next
commits will step by step empty remove_child_pid in the parent and move the
tasks to the helper.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>