Add a copy of OpenBSD's sys/queue.h as tor_queue.h

There are as many divergent implementations of sys/queue.h as there
are operating systems shipping it, it would seem.  They have some code
in common, but have drifted apart, and have added other stuff named
differently.  So I'm taking a relatively sane one, and hoping for the
best.

I'm taking OpenBSD's in particular because of the lack of external
dependencies, the presence of a CIRCLEQ (we could use one of those in
places), and the liberal licensing terms.

I'm naming the file tor_queue.h, since historically we've run into
trouble having headers with the same names as system headers (log.h,
for example.)