Port timers to OpenBSD, plus check for timer failures.

Port timers to OpenBSD, plus check for timer failures.

OpenBSD problem reported by Han Boetes.
* profiler.c (setup_cpu_timer): Check for failure of timer_settime
and/or setitimer.
(Fprofiler_cpu_stop): Don't assume HAVE_SETITIMER.
* syssignal.h (HAVE_ITIMERSPEC): New macro.  This is for platforms
like OpenBSD, which has timer_settime but does not declare it.
OpenBSD does not define SIGEV_SIGNAL, so use that when deciding
whether to use itimerspec-related primitives.  All uses of
HAVE_TIMER_SETTIME replaced with HAVE_ITIMERSPEC.