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/stdio/popen.c

@@ -33,10 +33,22 @@
 /**
  * Spawns subprocess and returns pipe stream.
  *
+ * The returned resource needs to be passed to pclose().
+ *
  * This embeds the Cosmopolitan Command Interpreter which provides
  * Bourne-like syntax on all platforms including Windows.
  *
- * @see pclose()
+ * @param cmdline is a unix shell script
+ * @param mode can be:
+ *     - `"r"` for reading from subprocess standard output
+ *     - `"w"` for writing to subprocess standard input
+ * @raise EINVAL if `mode` is invalid or specifies read+write
+ * @raise EMFILE if process `RLIMIT_NOFILE` has been reached
+ * @raise ENFILE if system-wide file limit has been reached
+ * @raise ECANCELED if thread was cancelled in masked mode
+ * @raise ENOMEM if we require more vespene gas
+ * @raise EAGAIN if `RLIMIT_NPROC` was exceeded
+ * @raise EINTR if signal was delivered
  * @cancellationpoint
  * @threadsafe
  */