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/intrin/getenv.c

@@ -34,10 +34,10 @@ char *getenv(const char *s) {
   if (!(p = environ)) return 0;
   e = _getenv(p, s);
 #if SYSDEBUG
-  if (!(s[0] == 'T' && s[1] == 'Z' && !s[2])) {
-    // TODO(jart): memoize TZ or something
-    STRACE("getenv(%#s) → %#s", s, e.s);
-  }
+  // if (!(s[0] == 'T' && s[1] == 'Z' && !s[2])) {
+  // TODO(jart): memoize TZ or something
+  STRACE("getenv(%#s) → %#s", s, e.s);
+  //}
 #endif
   return e.s;
 }