Enable PORTAGE_GRPNAME/USERNAME in make.conf.

Enable PORTAGE_GRPNAME/USERNAME in make.conf.

This is handy for prefix installs, since it allows these variables to
be set in make.conf rather than having them hardcoded. Eventually,
the corresponding portage.data constants may be deprecated in favor of
config attributes, since it's conceivable that multiple configurations
with different constants could be used simultaneously.