Better absolute path calculation in mp_get_playback_resume_config_filename

The problem is that mpv will generate different configs in `~/.mpv/watch_later` for one file:

* `mpv file.mkv` (`current/dir/file.mkv` will be used for hashing)
* `mpv ./file.mkv` (`current/dir/./file.mkv` will be used for hashing)
* `mpv ../dir/file.mkv` (`current/dir/../dir/file.mkv` will be used for hashing)

and so on.

I think solution is using `realpath` from `unistd.h` to get absolute file path, not result of joining current working directory and file name mpv was executed with.

Variable `realpath` was renamed to `real_path` to avoid conflict with this function.