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/closefrom.c

@@ -37,6 +37,7 @@
  * @raise EBADF on OpenBSD if `first` is greater than highest fd
  * @raise EINVAL if flags are bad or first is greater than last
  * @raise EMFILE if a weird race condition happens on Linux
+ * @raise ECANCELED if thread was cancelled in masked mode
  * @raise EINTR possibly on OpenBSD
  * @raise ENOMEM on Linux maybe
  */