One may now specify an alternative stage1 in a style like run and fetch:
rkt run --stage1-image foo.com/rocket/stage1 app
--stage1-image defaults to "stage1.aci" within the same directory as the rkt
binary. This is discovered at runtime via "/proc/self/exe"; as long as the rkt
executable and stage1.aci share a directory it should "just work" regardless of
the directory's location and where rkt is executed from.
