Use high-resolution timebases for select().

Use high-resolution timebases for select().

* select.h: Change prototype for select_stuff::wait() for larger
  microsecond timeouts.
* select.cc (pselect): Convert from old cygwin_select().
  Implement microsecond timeouts.
  (cygwin_select): Rewrite as a wrapper on pselect().
  (select): Implement microsecond timeouts.
  (select_stuff::wait): Implement microsecond timeouts with a timer
  object.