ReverseIP: add reverse lookup for bare IP4 addresses.

There are some potential problems with this:

- DNS lookup can be slow, although we try to mitigate this by only
  waiting for up to 1 second for a response.
- It's more or less uncacheable in the application, given the wide
  search space and unknown cache invalidation parameters.
- Results will depend on the resolver set up.  This may include leaking
  some information about specialized internal DDG reverse lookup, if any.
- Returns FQDN, which is both correct and possibly not exactly what the
  searcher might expect.

This version only handles well-formed IP4 addresses.  It could be easily
expanded to include IP6, but I wanted to make sure this was what was
being sought before adding more matching.