Follow-on to #1935 - Ensure ffmpeg is executable, remove possible doubled "/"

- movie::find_ffmpeg - made it use is_executable instead of just file_exists.
- system::find_binary - removed possible doubled "/" in paths.

This should go in 3.0.x, too, as I've seen a couple people get confused when we report that it's found but it doesn't work since their host doesn't allow execution of user-uploaded binaries.