s3-prefork: do not use a lock_fd, just race on accept()

s3-prefork: do not use a lock_fd, just race on accept()

We used a lock mimicking what apache does for preforked children.
But it doesn't work properly in our case because we do not stop once a request
has been served. Clients are allowed to perform multiple requests and keep the
connection open.
This means that if we allow multiple clients per children, then a child could
take the lock and then be asked to do a long or even locking operation by a
client it already is serving. This woulkd cause the whole server to deadlock,
as the child is now busy and also holding on the lock.
Using a race on accept() by having a tevent_fd on the listening socket wait
for read events we never deadlock. At most we cause a bit of contention among
children. But in the generic case connections are much less frequent for us as
clients tend to be long lived. So the little contention we may have is not a
big deal.

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