* Use `packed struct` for flags arguments.  So, instead of
  `linux.FUTEX.WAIT` use `.{ .cmd = .WAIT, .private = true }`

* rename `futex_wait` and `futex_wake` which didn't actually specify
  wait/wake, as `futex_3arg` and `futex_4arg` (as its the number
  of parameters that is different, the `op` is whatever is specified.

* expose the full six-arg flavor of the syscall (for some of the advanced
  ops), and add packed structs for their arguments.

* Use a `packed union` to support the 4th parameter which is sometimes a
  `timespec` pointer, and sometimes a `u32`.

* Add tests that make sure the structure layout is correct and that the
  basic argument passing is working (no actual futexes are contended).