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/intrin/pthread_key_create.c

@@ -29,6 +29,8 @@
  * key's value is nonzero. The key's value is set to zero before it gets
  * called. The ordering for multiple destructor calls is unspecified.
  *
+ * The result should be passed to pthread_key_delete() later.
+ *
  * @param key is set to the allocated key on success
  * @param dtor specifies an optional destructor callback
  * @return 0 on success, or errno on error