MINOR: dns: Implement `parse-resolv-conf` directive

MINOR: dns: Implement `parse-resolv-conf` directive

This introduces a new directive for the `resolvers` section:
`parse-resolv-conf`. When present, it will attempt to add any
nameservers in `/etc/resolv.conf` to the list of nameservers
for the current `resolvers` section.

[Mailing list thread][1].

[1]: https://www.mail-archive.com/haproxy@formilux.org/msg29600.html