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)

test/libc/thread/sem_open_test.c

@@ -103,7 +103,7 @@ TEST(sem_open, test) {
 TEST(sem_close, withUnnamedSemaphore_isUndefinedBehavior) {
   if (!IsModeDbg()) return;
   sem_t sem;
-  ASSERT_SYS(0, 0, sem_init(&sem, 1, 0));
+  ASSERT_SYS(0, 0, sem_init(&sem, 0, 0));
   SPAWN(fork);
   IgnoreStderr();
   sem_close(&sem);