CVE-2016-2110: auth/ntlmssp: call ntlmssp_sign_init if we provide GENSEC_FEATURE_SIGN

It's important to check if got the GENSEC_FEATURE_SIGN and if the caller
wanted it.

The caller may only asked for GENSEC_FEATURE_SESSION_KEY which implicitly
negotiates NTLMSSP_NEGOTIATE_SIGN, which might indicate GENSEC_FEATURE_SIGN
to the SPNEGO glue code.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11644

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