Allow SPNEGO fallback to NTLM without mechlistMIC

For interoperability with Windows Server 2003 and earlier, loosen the
initiator's enforcement of RFC 4178's mechlistMIC requirement when
falling back to NTLMSSP.

[ghudson@mit.edu: rewrote commit message, added comment to NTLMSSP
OID]

ticket: 7975
target_version: 1.13
tags: pullup