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/runtime/cocmd.c

@@ -309,10 +309,10 @@ static int Usleep(void) {
   struct timespec t;
   if (n > 1) {
     f = 0;
-    t = _timespec_frommicros(atoi(args[1]));
+    t = timespec_frommicros(atoi(args[1]));
   } else {
     f = TIMER_ABSTIME;
-    t = _timespec_max;
+    t = timespec_max;
   }
   return clock_nanosleep(0, f, &t, 0);
 }