Initial commit of cross-platform root-shell validity promise.

The root shell must be on the root filesystem, since otherwise
single-user mode doesn't work. This promise insures that the
root shell is not located on a mounted filesystem, and that
the directory in which it is found is not a symlink.
Check default shell for validity, other minor improvements.
Parameterization of module bundle.
Preliminaries of cross-platform password expiration enforcement.
Merge branch 'master' of ssh://github.com/mvpel/design-center into software_demo