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/thread/pthread_kill.c

@@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/calls/syscall_support-sysv.internal.h"
 #include "libc/errno.h"
 #include "libc/thread/posixthread.internal.h"
 #include "libc/thread/thread.h"
@@ -34,7 +35,7 @@
 errno_t pthread_kill(pthread_t thread, int sig) {
   int rc, e = errno;
   struct PosixThread *pt = (struct PosixThread *)thread;
-  if (!tkill(pt->tid, sig)) {
+  if (!__tkill(pt->tid, sig, pt->tib)) {
     rc = 0;
   } else {
     rc = errno;