Use nanosleep(2) in pg_usleep, if available?

In the thread about vacuum_cost_delay vs vacuum_cost_limit,
I wondered whether nanosleep(2) would provide any better
timing resolution than select(2).  Some experimentation
suggests that it doesn't, but nonetheless I see a good reason
why we should consider making pg_usleep use nanosleep() if
possible: it's got much better-defined semantics for what
happens if a signal arrives.  The comments for pg_usleep
lay out the problems with relying on select():

 * CAUTION: the behavior when a signal arrives during the sleep is platform
 * dependent.  On most Unix-ish platforms, a signal does not terminate the
 * sleep; but on some, it will (the Windows implementation also allows signals
 * to terminate pg_usleep).  And there are platforms where not only does a
 * signal not terminate the sleep, but it actually resets the timeout counter
 * so that the sleep effectively starts over!  It is therefore rather hazardous
 * to use this for long sleeps; a continuing stream of signal events could
 * prevent the sleep from ever terminating.  Better practice for long sleeps
 * is to use WaitLatch() with a timeout.

While the WaitLatch alternative avoids the problem, I doubt
we're ever going to remove pg_usleep entirely, so it'd be
good if it had fewer sharp edges.  nanosleep() has the
same behavior as Windows, ie, the sleep is guaranteed to be
terminated by a signal.  So if we used nanosleep() where available
we'd have that behavior on just about every interesting platform.

nanosleep() does exist pretty far back: it's in SUSv2, though
that version of the standard allows it to fail with ENOSYS.
Not sure if we'd need to teach configure to check for that
possibility.

Thoughts?

			regards, tom lane