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)

test/libc/intrin/strsignal_r_test.c

@@ -34,5 +34,6 @@ TEST(strsignal, test) {
 
 TEST(strsignal, realtime) {
   if (!SIGRTMIN) return;
+  EXPECT_STREQ("SIGTHR", strsignal(SIGTHR));
   ASSERT_STREQ("SIGRTMIN+1", strsignal(SIGRTMIN + 1));
 }