auth/gensec: make sure gensec_start_mech_by_authtype() resets SIGN/SEAL before starting

auth/gensec: make sure gensec_start_mech_by_authtype() resets SIGN/SEAL before starting

We want to set GENSEC_FEATURE_SIGN and GENSEC_FEATURE_SEAL based on the given
auth_level and should not have GENSEC_FEATURE_SEAL if
DCERPC_AUTH_LEVEL_INTEGRITY is desired.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11061

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>