CVE-2016-2111: auth/gensec: correctly report GENSEC_FEATURE_{SIGN,SEAL} in schannel_have_feature()

This depends on the DCERPC auth level.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11749

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
