source4/smbd: refactor the process model for prefork

 source4/smbd: refactor the process model for prefork

    Refactor the process model code to allow the addition of a prefork
    process model.

    - Add a process context to contain process model specific state
    - Add a service details structure to allow service to indicate which
      process model options they can support.

    In the new code the services advertise the features they support to the
    process model.  The process model context is plumbed through to allow the
    process model to keep track of the supported options, and any state
    the process model may require.

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