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/sigwaitinfo.c

@@ -25,6 +25,7 @@
  * @param info if not null shall receive info about signal
  * @return signal number on success, or -1 w/ errno
  * @raise EINTR if an asynchronous signal was delivered instead
+ * @raise ECANCELED if thread was cancelled in masked mode
  * @raise ENOSYS on OpenBSD, XNU, and Windows
  * @see sigtimedwait()
  * @cancellationpoint