glue: Add comment that argv_save and __main_thread_id need not be set here.

But they still are so the glue code is compatible with older Haiku versions.

src/system/glue/start_dyn.c

@@ -32,6 +32,7 @@ _start(int argc, char **argv, char **environment)
 {
 	int returnCode;
 
+	// These are kept here to make our glue code usable under older Haiku versions.
 	argv_save = argv;
 	__main_thread_id = find_thread(NULL);