Change queue_head and queue_tail to have rr prefix

Currently they are not a problem (since they are static) but if they
are made non-static in the future (for example, see one such attempt
in commit e7fd59b5b0c6e08fda92ec6804a06e1021f34ebd) queue_head will
collide with some other symbol in a way that causes *very* strange
runtime bugs.