Use timer_create() for the interval timer, if available
This lets the threaded RTS use SIGVTALRM rather than SIGALRM for its interval timer signal, so the threaded and non-threaded RTS are compatible. It unfortunately doesn't completely fix #850/#1156, for that we really have to use a restartable sleep instead of usleep(). Also I cleaned up the timer API a little: instead of returning an error value that ultimately gets ignored, we now report errors from system calls and exit.
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