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/posix_spawn.c

@@ -68,7 +68,6 @@ static int RunFileActions(struct _posix_faction *a) {
  * @param envp is environment variables, or `environ` if null
  * @return 0 on success or error number on failure
  * @see posix_spawnp() for `$PATH` searching
- * @cancellationpoint
  * @tlsrequired
  * @threadsafe
  */
@@ -81,9 +80,6 @@ int posix_spawn(int *pid, const char *path,
   int s, child, policy;
   struct sched_param param;
   struct sigaction dfl = {0};
-  if (_weaken(pthread_testcancel)) {
-    _weaken(pthread_testcancel)();
-  }
   if (!(child = vfork())) {
     if (attrp && *attrp) {
       posix_spawnattr_getflags(attrp, &flags);