Correct OPAM post-message for vim plugin.

- Fixes path to use correct path ``/vim/syntax/ocp-indent.vim``, like ``Makefile``.
- Fixes vim invalid syntax on L25.
- Filters newline returned from ``system("opam config var share")``.
- Retains ``opam config var share`` environment.

Output:

![2014-01-09-095326_5206x1080_scrot](https://f.cloud.github.com/assets/26336/1874451/f2d27a8c-78d0-11e3-8ad4-a64763200881.png)