Use EROOT instead of ROOT for keys everywhere.

Use EROOT instead of ROOT for keys everywhere.

It makes more sense to use EROOT instead of ROOT for keys in mappings
like portage.db, since it allows for multiple prefixes to exist
simultaneously within the same map without having a key collision.
This affects all portageq commands which take a <root> parameter, since
that parameter now corresponds to EROOT instead of ROOT. None of this
makes any difference for non-prefix users, since EROOT and ROOT are
identical when EPREFIX is empty.