Implement support for POSIX thread cancellations

This change makes some miracle modifications to the System Five system
call support, which lets us have safe, correct, and atomic handling of
thread cancellations. It all turned out to be cheaper than anticipated
because it wasn't necessary to modify the system call veneers. We were
able to encode the cancellability of each system call into the magnums
found in libc/sysv/syscalls.sh. Since cancellations are so waq, we are
also supporting a lovely Musl Libc mask feature for raising ECANCELED.

libc/thread/pthread_join.c

@@ -25,9 +25,12 @@
  * Waits for thread to terminate.
  *
  * @param value_ptr if non-null will receive pthread_exit() argument
+ *     if the thread called pthread_exit(), or `PTHREAD_CANCELED` if
+ *     pthread_cancel() destroyed the thread instead
  * @return 0 on success, or errno with error
  * @raise EDEADLK if `thread` is the current thread
  * @raise EINVAL if `thread` is detached
+ * @cancellationpoint
  * @returnserrno
  * @threadsafe
  */