_gss_ntlm_accept_sec_context() does not provide the acceptor domain to
nsi_probe(); for consistency, _gss_ntlm_acquire_cred_from() should do that
same. Providing the acceptor domain was breaking tests.
Note that the Heimdal NTLM implementation is old and has few consumers (Apple
and Samba use their own implementations). Arguably we should merge the Apple
implementation or remove it.