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/stdio/tmpfile.c

@@ -54,6 +54,9 @@
  * is Linux-only and will cause open() failures on all other platforms.
  *
  * @see tmpfd() if you don't want to link stdio/malloc
+ * @raise ECANCELED if thread was cancelled in masked mode
+ * @raise EINTR if signal was delivered
+ * @cancellationpoint
  * @asyncsignalsafe
  * @threadsafe
  * @vforksafe