Avoid treating 'sh -c emacs' as an instance of emacs, by recognising

that the -c means that the next argument isn't the name of a script.
Likewise with various other interpreter programs.

This is unpleasantly complicating the mechanism I'll undoubtedly end
up needing for overriding the built-in interpreter list, but there we
go.

git-svn-id: svn://svn.tartarus.org/sgt/utils@9717 cda61777-01e9-0310-a592-d414129be87e