sanitize whitespace
rename global variables to be obvious
rename MPI_PORTDIR and pull it from the portage env
empi:  allow all users to see usage screen
empi:  provide and set overlay default

Set default overlay path to /var/cache/overlays/mpi
empi:  manage overlay metadata

Add overlay metadata management.  Snagged from crossdev.
http://git.overlays.gentoo.org/gitweb/?p=proj/crossdev.git