Expose separate function to determine if thread is attached. Revert unity...

Expose separate function to determine if thread is attached. Revert unity specific change to prevent mono_thread_current from auto attaching.

mono/metadata/threads.c

@@ -1299,14 +1299,28 @@ mono_thread_current (void)
 
 	current_thread_ptr = get_current_thread_ptr_for_domain (domain, internal);
 
-// UNITY: we don't want to attach just because we call mono_thread_current
-//	if (!*current_thread_ptr) {
-//		g_assert (domain != mono_get_root_domain ());
-//		*current_thread_ptr = new_thread_with_internal (domain, internal);
-//	}
+	if (!*current_thread_ptr) {
+		g_assert (domain != mono_get_root_domain ());
+		*current_thread_ptr = new_thread_with_internal (domain, internal);
+	}
 	return *current_thread_ptr;
 }
 
+MonoBoolean
+mono_unity_thread_is_attached ()
+{
+	MonoDomain *domain = mono_domain_get ();
+        MonoInternalThread *internal = mono_thread_internal_current ();
+        MonoThread **current_thread_ptr;
+
+        if (!domain || !internal)
+        return FALSE;
+
+        current_thread_ptr = get_current_thread_ptr_for_domain (domain, internal);
+
+	return *current_thread_ptr != NULL;
+}
+
 MonoInternalThread*
 mono_thread_internal_current (void)
 {