Abortable curl_easy_perform()

1. Introduce new option to curl_share_setopt(): CURL_LOCK_ABORT
2. Participating transfers must then use a shared object with that bit set
3. Such share objects should have the mutex callback set.

curl_easy_abort()

CURLcode curl_easy_abort(CURL *easy);

Can be called from the same or another thread. This aborts the specific transfer iff that transfer was setup to allow being externally aborted like described above. A transfer that is aborted is terminated, not simply paused.

Trying to abort a non-participating transfer will only result in a CURLE_ABORT_DENIED (new) return code.

A transfer that is successfully stopped by this function will return the CURLE_ABORTED (new) return code.

Users of this function must take precautions to make sure that the handle itself is still valid when used in this function.