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/creat.c

@@ -31,6 +31,7 @@
  * @param mode is octal bits, e.g. 0644 usually
  * @return file descriptor, or -1 w/ errno
  * @see openat() for further documentation
+ * @cancellationpoint
  * @asyncsignalsafe
  * @restartable
  * @threadsafe