s4:auth/gensec: let GENSEC_FEATURE_SESSION_KEY result in GSS_C_INTEG_FLAG

s4:auth/gensec: let GENSEC_FEATURE_SESSION_KEY result in GSS_C_INTEG_FLAG

This is important to allow the 'new_spnego' with mech_list protection to work
for a SMB session setup.

This is not strictly needed as we always announce GENSEC_FEATURE_SESSION_KEY
in gensec_gssapi_have_feature(), but it's better to send GSS_C_INTEG_FLAG
over the wire.

This may prevent a ticket from a Samba client to an SMB server
(particularly a DC) being misused to connect to the LDAP server on that
DC, as the LDAP server will require GSSAPI signing of the connection.

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