Fix prelude-open-with

Currently, `prelude-open-with` does not work with files whose name has a
space or non-ascii character in it. I guess `shell-quote-argument` is
not needed for `start-process`.

I also refactor the code a little bit, and since we dot not care about
the output of `open`, so I removed the buffer name passed to `start-process`
Merge pull request #669 from toctan/fix-prelude-open-with

Fix prelude-open-with