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/thread/pthread_cond_timedwait.c

@@ -27,7 +27,7 @@
  * Waits for condition with optional time limit, e.g.
  *
  *     struct timespec ts;  // one second timeout
- *     ts = _timespec_add(_timespec_real(), _timespec_frommillis(1000));
+ *     ts = timespec_add(timespec_real(), timespec_frommillis(1000));
  *     if (pthread_cond_timedwait(cond, mutex, &ts) == ETIMEDOUT) {
  *       // handle timeout...
  *     }