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/sock/epoll.c

@@ -1499,6 +1499,7 @@ int epoll_ctl(int epfd, int op, int fd, struct epoll_event *ev) {
  * @param maxevents is array length of events
  * @param timeoutms is milliseconds, 0 to not block, or -1 for forever
  * @return number of events stored, 0 on timeout, or -1 w/ errno
+ * @cancellationpoint
  * @norestart
  */
 int epoll_wait(int epfd, struct epoll_event *events, int maxevents,