Make Time::HiRes::sleep() and usleep() to return

Make Time::HiRes::sleep() and usleep() to return
the number of seconds (a floating point value) and
microseconds (an integer) actually slept (well,
modulo the time spent in measuring the time actually
slept...), to be a better drop-in replacement for
the builtin sleep().  Noticed by Chris Nandor.

Also make usleep() to croak if fed illegal number
of useconds (anything greater than 1_000_000).

p4raw-id: //depot/perl@12449