Filter prefix variables ED, EPREFIX, and EROOT inside

Filter prefix variables ED, EPREFIX, and EROOT inside
filter_readonly_variables() but don't mark them readonly for now since some
ebuilds/eclasses try to assign EROOT (such as kde4-base_pkg_setup) which
causes the whole bash shell to exit like this:

>>> Extracting info
 * CPV:  kde-base/oxygen-icons-4.3.4
 * REPO: gentoo
 * USE:  elibc_glibc kernel_linux test userland_GNU x86
/var/tmp/portage/kde-base/oxygen-icons-4.3.4/temp/environment: line 2781: EROOT: readonly variable

Also, add ED to the environment blacklist. (trunk r15030)

svn path=/main/branches/2.1.7/; revision=15064