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_setcancelstate.c

@@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/blockcancel.internal.h"
 #include "libc/errno.h"
 #include "libc/thread/posixthread.internal.h"
 #include "libc/thread/thread.h"
@@ -76,3 +77,13 @@ errno_t pthread_setcancelstate(int state, int *oldstate) {
     return 0;
   }
 }
+
+int _pthread_block_cancellations(void) {
+  int oldstate;
+  pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &oldstate);
+  return oldstate;
+}
+
+void _pthread_allow_cancellations(int oldstate) {
+  pthread_setcancelstate(oldstate, 0);
+}