Setting DNS servers

[DBLouis]

Would you consider re-adding this functionality under a non-default cargo feature?

9cec74c624307f489d1a891fe7636876fc05ea6c

[sagebind]

I might consider restoring such an option under an optional feature. Since it only works when curl is compiled with c-ares support (which it typically isn't) it should retain a pretty clear warning label as such.

I was also not a fan of the previous warn-by-default behavior. If possible the API ought to error early if the option is not supported at runtime.

[DBLouis]

I made a PR at alexcrichton/curl-rust#427