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/dirstream.c

@@ -255,6 +255,8 @@ static textwindows dontinline struct dirent *readdir_nt(DIR *dir) {
  *
  * @returns newly allocated DIR object, or NULL w/ errno
  * @errors ENOENT, ENOTDIR, EACCES, EMFILE, ENFILE, ENOMEM
+ * @raise ECANCELED if thread was cancelled in masked mode
+ * @raise EINTR if we needed to block and a signal was delivered instead
  * @cancellationpoint
  * @see glob()
  */