-. decides where we chdir to before execing, we can't
sit where we are because our directory may not exist.
If not specified we go to '/'.
-s is used to source a rc script instead of execing cmd.
This is primarily to enable:
'#!/bin/auth/box -s'
Shebang line size is a bit tight(32), so we have this shorthand
to use rc along with setting up the required namespace components.