MFH: r448108

Change RESOLV from RADIO to SINGLE

cURL 7.55.1 use the threaded resolver (THREADED_RESOLVER) backend if c-ares
(CARES) is not available. That means you must enable either CARES or
THREADED_RESOLVER (default) option.

Reference: https://github.com/curl/curl/pull/1647

Approved by: ports-secteam (feld)