s3-prefork: Fix worker flags handling.

s3-prefork: Fix worker flags handling.

We can't have a clear idea of wether the worker is IDLE or BUSY.
The only things we can tell is if it is Alive, whether it is currently
Accepting connections or wether it is Exiting soon.

Remove PF_WORKER_IDLE, PF_WORKER_BUSY and replace their use with
PF_WORKER_ALIVE. Also properly assign PF_WORKER_ACCEPTING so that
users of the API can rely on the flag.

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