configure --enable-debug now enables -Wassign-enum with clang,
identifying several enum "abuses" also fixed.
Reported-by: Gisle Vanem
Bug: 879007f811 (commitcomment-42087553)Closes#5929
Added Curl_resolver_kill() for all three resolver modes, which only
blocks when necessary, along with test 1592 to confirm
curl_multi_remove_handle() doesn't block unless it must.
Closes#3428Fixes#3371
