s4:process_model Fix process_standard and process_onefork not to use

s4:process_model Fix process_standard and process_onefork not to use
multiple event contexts

It is NEVER valid to free an event context that anybody else may have
a reference to, and never normally valid to have two 'live' at once.
We must instead call tevent_re_initialise() to wipe clean an existing
pointer.

Andrew Bartlett