resolvers - resolv.conf fallback

Assuming no one else is already working on the feature, I'd like to
contribute
the change to have the resolvers section fall back to parsing
/etc/resolv.conf
when no nameserver directives are present.

This was previously discussed here: https://www.mail-
archive.com/haproxy@formilux.org/msg28626.html.
(Jim Freeman and I are colleagues.)

I've been surveying the code to see how it could be implemented. I think it
would make sense to register a 'post_section_parser' for the resolvers
section
that would add the nameservers from /etc/resolv.conf if no nameservers were
specified in the section. As Jim pointed out previously, libresolv could be
used
to parse the file. If that's undesirable for some reason, we could parse it
ourselves.

I'm new to the codebase, so I'm open to any suggestions or guidance anyone
may have.

Thanks,

Ben