unprivileged mode: fix cross-prefix support

In commit 1364fcd89384c9f60e6d72d7057dc00d8caba175, EROOT calculation
in portage.data did not account from cross-prefix support. This is
fixed by using new _target_root and _target_eprefix functions to
perform the calculation. The _target_eprefix function is also useful
in portageq, where the target EPREFIX needs to be known before
portage.settings is instantiated.

Fixes 1364fcd89384 ("Support unprivileged mode for bug #433453.")