pqueue: add info about not being thread safe
pqueue: fix make_room to not delete messages with higher priority

Old code would only check last element against the new priority
Also, make_room is now non-destructive when not enough room
pqueue: printf and documentation update
pqueue: queue_add now returns success value
pqueue: extra semicolon fixup
pqueue: add backoff test case, update documentation
abc: documentation update about what drivers requires root
abc: unamanged: only forward new messages instantly
abc: dont incremented backoff on send errors
abc: log error when queue is full
abc: only increment cycle counter when data was sent

Otherwise backed off messages are not guaranteed to
be sent the number of times wanted before dropping out.
abc: send packages with DO_NOT_CACHE flag exactly once