source4/smbd: add a prefork process model.

source4/smbd: add a prefork process model.

Add a pre fork process model to bound the number processes forked by
samba.  Currently workers are only pre-forked for the ldap server,  all
the other services have pre-fork support disabled.

When pre-fork support is disabled a new process is started for each
service, and requests are processed by that process.

This commit partially reverts commit
b5be45c453bd51373bade26c29828b500ba586ec.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>