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/nanosleep.c

@@ -28,6 +28,7 @@
  *     time when -1 w/ `EINTR` is returned otherwise `rem` is undefined
  * @return 0 on success, or -1 w/ errno
  * @raise EINVAL if `req->tv_nsec ∉ [0,1000000000)`
+ * @raise ECANCELED if thread was cancelled in masked mode
  * @raise EINTR if a signal was delivered and `rem` is updated
  * @raise EFAULT if `req` is NULL or `req` / `rem` is a bad pointer
  * @raise ENOSYS on bare metal