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/fdatasync-nt.c

@@ -23,5 +23,6 @@
 textwindows int sys_fdatasync_nt(int fd) {
   // TODO(jart): what should we do with worker pipes?
   if (!__isfdkind(fd, kFdFile)) return ebadf();
+  if (_check_interrupts(false, 0)) return -1;
   return FlushFileBuffers(g_fds.p[fd].handle) ? 0 : -1;
 }