gensec: move event context from gensec_*_init() to gensec_update()

gensec: move event context from gensec_*_init() to gensec_update()

This avoids keeping the event context around on a the gensec_security
context structure long term.

In the Samba3 server, the event context we either supply is a NULL
pointer as no server-side modules currently use the event context.

Andrew Bartlett

Signed-off-by: Stefan Metzmacher <metze@samba.org>