s4-smbd: don't initialise process models more than once

s4-smbd: don't initialise process models more than once

this also removes the event_context parameter from process model
initialisation. It isn't needed, and is confusing when a process model
init can be called from more than one place, possibly with different
event contexts.

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>