Add Vim portability shims
Add Vim portability shims

portable#test('-X', 'file') runs `test -X file` and returns 1 for
success or 0 for failure.

portable#owner('file') returns the username of the file.

benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles,benizi/dotfiles