Fix threads on Darwin 10.8

Fix threads on Darwin 10.8

 * We used to pun (64-bit) addresses into 32-bit mach_port_name.

 * We used to assume arbitrary 32-bit addresses were always
   acceptable mach port names.

 * Stop doing that. Instead, allocate small descriptors until one's
   address is an acceptable port name (inspired by CCL).

 * Also, keep a lock-free free-list of descriptors to skip the previous
   loop in common cases.

 * There are still some strange issues, but I can't tell if they're new,
   and they seem preferable to consistently lose-ing when spawning threads.