Rewrite gss_add_cred() (fix #413)

It turns out gss_add_cred() really needed a complete rewrite.  It's much
better to first have a gss_duplicate_cred() (which has been needed for
other reasons anyways), and use that when the input_cred_handle is not
GSS_C_NO_CREDENTIAL and output_cred_handle is not NULL, then mutate that
duplicate credential handle (or the input_cred_handle if
output_cred_handle is NULL).

lib/gssapi/ntlm/external.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2006 Kungliga Tekniska Högskolan
+ * Copyright (c) 2006-2018 Kungliga Tekniska Högskolan
  * (Royal Institute of Technology, Stockholm, Sweden).
  * All rights reserved.
  *
@@ -114,15 +114,16 @@ static gssapi_mech_interface_desc ntlm_mech = {
     NULL,
     ntlm_mo,
     sizeof(ntlm_mo) / sizeof(ntlm_mo[0]),
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
-    NULL,
+    NULL, /* gm_localname */
+    NULL, /* gm_authorize_localname */
+    NULL, /* gm_display_name_ext */
+    NULL, /* gm_inquire_name */
+    NULL, /* gm_get_name_attribute */
+    NULL, /* gm_set_name_attribute */
+    NULL, /* gm_delete_name_attribute */
+    NULL, /* gm_export_name_composite */
+    NULL, /* gm_duplicate_cred */
+    NULL, /* gm_compat */
 };
 
 gssapi_mech_interface