Implement forward compatible Prefix support. This makes EPREFIX, ED and

Implement forward compatible Prefix support.  This makes EPREFIX, ED and
EROOT available to the ebuild environment, and updates the manpage
accordingly.  EPREFIX is hardwired to the empty string, and not
configurable.  For that, the prefix branch should be used for now.

This should satisfy the requirements for EAPI=3. (trunk r15024)

svn path=/main/branches/2.1.7/; revision=15060