gss: port NegoEx implementation from MIT

An implementation of draft-zhu-negoex-04 for MIT Kerberos was developed in
2011. This has been recently integrated, with many fixes from Greg Hudson. This
commit ports it to Heimdal. The implementation has been interoperability tested
with MIT Kerberos and Windows, using the GSS EAP mechanism developed as part of
the Moonshot project.

The SPNEGO code was also updated to import the state machine from Apple which
improves mechListMIC processing and avoids discarding initial context tokens
generated during mechanism probing, that can be used for optimistic tokens.

Finally, to aid in testing, the GSS-API mechanism glue configuration file can
be changed using the environment variable GSS_MECH_CONFIG. This environment
variable name, along with the format of the configuration file, is compatible
with MIT (although it would be difficult for a single mechanism binary to
support both implementations).

lib/gssapi/mech/context.c

@@ -37,6 +37,7 @@
 #include "heim_threads.h"
 #include <krb5.h>
 #include "krb5_locl.h"
+#include "negoex_err.h"
 
 struct mg_thread_ctx {
     gss_OID mech;
@@ -99,6 +100,8 @@ _gss_mechglue_thread(void)
 	    return NULL;
 	}
 
+	krb5_add_et_list(ctx->context, initialize_ngex_error_table_r);
+
 	HEIMDAL_setspecific(context_key, ctx, ret);
 	if (ret) {
 	    krb5_free_context(ctx->context);
@@ -109,6 +112,16 @@ _gss_mechglue_thread(void)
     return ctx;
 }
 
+krb5_context
+_gss_mg_krb5_context(void)
+{
+    struct mg_thread_ctx *mg;
+
+    mg = _gss_mechglue_thread();
+
+    return mg ? mg->context : NULL;
+}
+
 OM_uint32
 _gss_mg_get_error(const gss_OID mech,
 		  OM_uint32 value,