Improve cancellations, randomness, and time

- Exhaustively document cancellation points
- Rename SIGCANCEL to SIGTHR just like BSDs
- Further improve POSIX thread cancellations
- Ensure asynchronous cancellations work correctly
- Elevate the quality of getrandom() and getentropy()
- Make futexes cancel correctly on OpenBSD 6.x and 7.x
- Add reboot.com and shutdown.com to examples directory
- Remove underscore prefix from awesome timespec_*() APIs
- Create assertions that help verify our cancellation points
- Remove bad timespec APIs (cmp generalizes eq/ne/gt/gte/lt/lte)

libc/calls/clock_nanosleep-xnu.c

@@ -31,10 +31,10 @@ int sys_clock_nanosleep_xnu(int clock, int flags, const struct timespec *req,
   struct timeval now, abs, rel;
   if (clock == CLOCK_REALTIME) {
     if (flags & TIMER_ABSTIME) {
-      abs = _timespec_totimeval(*req);
+      abs = timespec_totimeval(*req);
       sys_gettimeofday_xnu(&now, 0, 0);
-      if (_timeval_gt(abs, now)) {
-        rel = _timeval_sub(abs, now);
+      if (timeval_cmp(abs, now) > 0) {
+        rel = timeval_sub(abs, now);
         res = sys_select(0, 0, 0, 0, &rel);
       } else {
         res = 0;