Fix nonstandard TAILQ_FOREACH_REVERSE() definition

Fix nonstandard TAILQ_FOREACH_REVERSE() definition

Every current BSD system providing TAILQ_* macros define
TAILQ_FOREACH_REVERSE in this order:

TAILQ_FOREACH_REVERSE(var, head, field, headname)

However, libevent defines it in another order:

TAILQ_FOREACH_REVERSE(var, head, headname, field)

Here's a trivial patch to have libevent compatible with stock queue.h headers.

-Frank.

[From sourceforge patch 2995179. codesearch.google.com confirms that
the only people defining TAILQ_FOREACH_REVERSE our way are people
using it in a compatibility header like us.  Did we copy this from
OpenSSH or something?]

-Nick