oauth: Remove expired timers from the multiplexer

oauth: Remove expired timers from the multiplexer

In a case similar to the previous commit, an expired timer can remain
permanently readable if Curl does not remove the timeout itself. Since
that removal isn't guaranteed to happen in real-world situations,
implement drain_timer_events() to reset the timer before calling into
drive_request().

Moving to drain_timer_events() happens to fix a logic bug in the
previous caller of timer_expired(), which treated an error condition as
if the timer were expired instead of bailing out.

The previous implementation of timer_expired() gave differing results
for epoll and kqueue if the timer was reset. (For epoll, a reset timer
was considered to be expired, and for kqueue it was not.) This didn't
previously cause problems, since timer_expired() was only called while
the timer was known to be set, but both implementations now use the
kqueue logic.

Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Backpatch-through: 18
Discussion: https://postgr.es/m/CAOYmi+nDZxJHaWj9_jRSyf8uMToCADAmOfJEggsKW-kY7aUwHA@mail.gmail.com