Relax dns search to accept empty domain

So now dns options can achieve all these scenarios related to `search` instruction on `resolv.conf`:

- Use host `resolv.conf` search, if no option is used.
- Use a custom search if `--dns-search=mysearch` is used.
- Do not include a search on `resolv.conf` if `--dns-search=''` is used.

The third scenario is the fix.