Make fixes and improvements

- clock_nanosleep() is now much faster on OpenBSD and NetBSD
- Thread joining is now much faster on NetBSD
- FreeBSD timestamps are now more accurate
- Thread spawning now goes faster on XNU
- Clean up the clone() code

libc/calls/timespec_mono.c

@@ -29,6 +29,6 @@
  */
 struct timespec timespec_mono(void) {
   struct timespec ts;
-  _npassert(!clock_gettime(CLOCK_MONOTONIC_FAST, &ts));
+  _npassert(!clock_gettime(CLOCK_MONOTONIC, &ts));
   return ts;
 }