ds: rely on autovivication for waitpid bits

No need to create an arrayref until we need it, and fix up a
comment while we're in the area.  Some aesthetic changes while
we're at it:

- Rename $WaitPids to $wait_pids to make it clear this is
  unique to our implementation and not in Danga::Socket.

- rewrite dwaitpid() to reduce indentation level