curl: Fixed usage of DNS options when not using c-ares resolver

curl: Fixed usage of DNS options when not using c-ares resolver

Commit 32352ed6adddcb introduced various DNS options, however, these
would cause curl to exit with CURLE_NOT_BUILT_IN when c-ares wasn't
being used as the backend resolver even if the options weren't set
by the user.

Additionally corrected some minor coding style errors from the same
commit.