CVE-2016-2110: auth/gensec: add gensec_may_reset_crypto() infrastructure

[MS-SPNG] requires the NTLMSSP RC4 states to be reset after
the SPNEGO exchange with mechListMic verification (new_spnego).

This provides the infrastructure for this feature.

The 'reset_full' parameter is needed to support the broken
behavior that windows only resets the RC4 states but not the
sequence numbers. Which means this functionality is completely
useless... But we want to work against all windows versions...

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>
