The http transport (with curl-multi option, which is the default
these days) failed to remove curl-easy handle from a curlm session,
which led to unnecessary API failures.

* ew/http-do-not-forget-to-call-curl-multi-remove-handle:
  http: always remove curl easy from curlm session on release
  http: consolidate #ifdefs for curl_multi_remove_handle
  http: warn on curl_multi_add_handle failures