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.
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