[krbdev.mit.edu #3775] krb5_gss_accept_sec_context should handle inconsistent mutual auth requests

[Tom Yu via RT]

If an initiator sends an initial krb5 mechanism token with GSS_C_MUTUAL_FLAG clear, but with 
mutual-required set in the AP-REQ, krb5_gss_accept_sec_context() only looks at the GSS flag.  
The MS krb5 GSS mechanism implementation, when mutual auth isn't requested, appears to 
emit a krb5 token that is inconsistent in this way, yet expects a reply token.