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/sysinfo.c

@@ -40,7 +40,7 @@ static int64_t GetUptime(void) {
   size_t n = sizeof(x);
   int mib[] = {CTL_KERN, KERN_BOOTTIME};
   if (sys_sysctl(mib, ARRAYLEN(mib), &x, &n, 0, 0) == -1) return 0;
-  return _timespec_real().tv_sec - x.tv_sec;
+  return timespec_real().tv_sec - x.tv_sec;
 }
 
 static int64_t GetPhysmem(void) {