Remaining changes to implement the prefork process model

Remaining changes to implement the prefork process model

To use, run 'smbd -M prefork'

By default, only the smb service is preforked.  4 children are
created, and all listen for new connections.  The Linux Kernel 'wake
one' behaviour should ensure that only one is given the oportunity to
accept.  We need to look into the ideal number of worker children, as
well as load balancing behaviours.

To change, set:

prefork children : smb = 6

valid service names (smb in this example) match those in 'server services'.

Andrew Bartlett and David Disseldorp
(This used to be commit 35313c0aa3fbfdd943edfb7bafd7645b1a0c54e9)