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)

examples/clock_getres.c

@@ -28,7 +28,7 @@ void show(int clock) {
   }
   shown[n++] = clock;
   if (clock_getres(clock, &ts) != -1) {
-    kprintf("%s %'ld ns\n", DescribeClockName(clock), _timespec_tonanos(ts));
+    kprintf("%s %'ld ns\n", DescribeClockName(clock), timespec_tonanos(ts));
   } else {
     kprintf("%s %s\n", DescribeClockName(clock), _strerrno(errno));
   }