In b0635bf, I added an early header check to the Meson OAuth support,
which was intended to duplicate the later checks for
HAVE_SYS_[EVENT|EPOLL]_H. However, I implemented the new test via
check_header() -- which tries to compile -- rather than has_header(),
which just looks for the file's existence.

The distinction matters on OpenBSD, where <sys/event.h> can't be
compiled without including prerequisite headers, so -Dlibcurl=enabled
failed on that platform. Switch to has_header() to fix this.

If a socket is added to the kqueue, becomes readable/writable, and
subsequently becomes non-readable/writable again, the kqueue itself will
remain readable until either the socket registration is removed, or the
stale event is cleared via a call to kevent().

In many simple cases, Curl itself will remove the socket registration
quickly, but in real-world usage, this is not guaranteed to happen. The
kqueue can then remain stuck in a permanently readable state until the
request ends, which results in pointless wakeups for the client and
wasted CPU time.

Implement drain_socket_events() to call kevent() and unstick any stale
events. This is called right after drive_request(), before we return
control to the client to wait. To make sure we've taken a look at the
entire queue, register_socket() now tracks the number of outstanding
registrations.

Suggested-by: Thomas Munro <thomas.munro@gmail.com>

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.

Tracking down the bugs that led to the addition of drain_socket_events()
and drain_timer_events() was difficult, because an inefficient flow is
not visibly different from one that is working properly. To help
maintainers notice when something has gone wrong, track the number of
calls into the flow as part of debug mode, and print the total when the
flow finishes.

A new test makes sure the total count is less than 100. (We expect
something on the order of 10.) This isn't foolproof, but it is able to
catch several regressions in the logic of the prior two commits, and
future work to add TLS support to the oauth_validator test server should
strengthen it as well.

To better record the internal behaviors of oauth-curl.c, add a unit test
suite for the socket and timer handling code. This is all based on TAP
and driven by our existing Test::More infrastructure.