A command that lets you start another command with any argv[0].


`exec-as bash -bash` will run the first program in $PATH
named bash, and set argv[0] to -bash (making it a login
shell.) You can add addition argument as needed.


Note that to actually exec rather than then fork–exec:ing
you need to type `exec exec-as` instead of `exec-as`, as
the shell would otherwise fork first.