http: optimize HTTP request thread we don't need a mutex at all, in fact all we need is an atomic boolean to signify whether the thread is cancelled. curl options are now for the most part handled by a separate function to keep them in sync between non-threaded and threaded implementations

These icons are from Yusuke Kamiyamane's Fugue Icons[1] pack.
There are no changes divergent from any of the original files.

[1]: https://p.yusukekamiyamane.com/