fix theslinux#143: pacman doesn't encode URI

pacman doesn't encode URI before sending request to server,
hence pacman can't download package with special character
in their name (e.g, pacman%makepkg).

We fix the issue by (curl_easy_escape) in (curl_set_handle_opts);
after that we call (curl_easy_unescape) to have the correct
local filename.