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)

net/turfwar/blackholed.c

@@ -314,8 +314,7 @@ void UseLog(void) {
 }
 
 void UninterruptibleSleep(int ms) {
-  struct timespec ts =
-      _timespec_add(_timespec_real(), _timespec_frommillis(ms));
+  struct timespec ts = timespec_add(timespec_real(), timespec_frommillis(ms));
   while (clock_nanosleep(CLOCK_REALTIME, TIMER_ABSTIME, &ts, 0)) errno = 0;
 }