JNI: marking and unmarking global and local references

[mgd at swarm dot org (Marcus G. Daniels)]

JNI says that local and global references are different categories,
but the mark_for_gc and unmark_for_gc conflate them.  For example, the
GCJ libjava runtime will undo the intent of the NewGlobalRef below
(create a unique handle to a preserved global object), since gref and
lref end up having the same value.

  jobject lref = (*env)->GetObjectClass (env, obj);
  jobject gref = (*env)->NewGlobalRef (env, lref);
  (*env)->DeleteLocalRef (env, lref)