auth/gensec: add a gensec_update_ev() function

auth/gensec: add a gensec_update_ev() function

This is the current gensec_update() which takes an optional
tevent_context structure and allows semi-async code.

This is just a temporary solution on the way to kill
the semi-async code completely, by using gensec_update_send/recv.

By providing a gensec_update_ev(), we can remove the explicit
tevent_context from gensec_update() and fix all the sane callers.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>