Do not loop on add_cred_from and other new methods

Several new GSS-API methods were added but GSSAPI_ADD_METHOD was
called to add them rather than GSSAPI_ADD_METHOD_NOLOOP.  This means
that the implementation from the GSS-API mechglue would be used if the
mechanism had no implementation.  As a result, the mechglue will call
into itself exhausting the call stack in an endless loop when one of
these methods is called.