MultiHttpClient: Also fallover to non-curl if curl_multi* is blocked

Requested by a user at https://www.mediawiki.org/wiki/Topic:Vkk1ahk3eggd9747 for
whom their hoster provides curl but with multi-threaded functions removed for
some reason.

Change-Id: Id3877c600ae02feffb67f74a815430f8e679230a