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/calls/wait4.c

@@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/calls/cp.internal.h"
 #include "libc/calls/struct/rusage.internal.h"
 #include "libc/calls/wait4.h"
 #include "libc/dce.h"
@@ -41,6 +42,8 @@
 int wait4(int pid, int *opt_out_wstatus, int options,
           struct rusage *opt_out_rusage) {
   int rc, ws = 0;
+  BEGIN_CANCELLATION_POINT;
+
   if (IsAsan() &&
       ((opt_out_wstatus &&
         !__asan_is_valid(opt_out_wstatus, sizeof(*opt_out_wstatus))) ||
@@ -53,6 +56,8 @@ int wait4(int pid, int *opt_out_wstatus, int options,
     rc = sys_wait4_nt(pid, &ws, options, opt_out_rusage);
   }
   if (rc != -1 && opt_out_wstatus) *opt_out_wstatus = ws;
+
+  END_CANCELLATION_POINT;
   STRACE("wait4(%d, [%#x], %d, %p) → %d% m", pid, ws, options, opt_out_rusage,
          rc);
   return rc;