Simplify timerfd configuration and fix some minor glitches.

Simplify timerfd configuration and fix some minor glitches.

* configure.ac (HAVE_TIMERFD): Define only if TFD_CLOEXEC works,
since the code leaked file descriptors to children when !TFD_CLOEXEC.
(HAVE_TIMERFD_CLOEXEC): Remove; no longer used.
* m4/clock_time.m4 (gl_CLOCK_TIME): Don't check for clock_getres.
This reverts the previous change to this file, so it matches
gnulib again.
* src/atimer.c (TIMERFD_CREATE_FLAGS): Remove; we now assume TFD_CLOEXEC.
(alarm_timer, alarm_timer_ok, set_alarm, init_atimer):
Fall back on timer_create if timerfd_create fails at runtime.
(resolution) [HAVE_CLOCK_GETRES]: Remove; we now rely on the
kernel primitives to do resolution.  All uses removed.
(timerfd) [!HAVE_TIMERFD]: Define to be -1, for convenience.
(turn_on_atimers): Clear timer_create-based timers too,
for consistency.