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/spnego/external.c

@@ -1,5 +1,7 @@
 /*
  * Copyright (c) 2004, PADL Software Pty Ltd.
+ * Copyright (c) 2018 Kungliga Tekniska Högskolan
+ * (Royal Institute of Technology, Stockholm, Sweden).
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -136,15 +138,16 @@ static gssapi_mech_interface_desc spnego_mech = {
     NULL,
     spnego_mo,
     sizeof(spnego_mo) / sizeof(spnego_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