create_trees: whitelist PORTAGE_DEPCACHEDIR

This fixes PORTAGE_DEPCACHEDIR to work for ROOT=/ when working with a
different target ROOT.
Support unprivileged mode for bug #433453.

This takes the existing unprivileged prefix support and enables it to
work when EPREFIX is empty. This "unprivileged" is automatically
enabled if the current user is not root, but has write access to the
EROOT directory (not due to the 0002 bit). For use in conditional logic
(for example, see fowners diff), "unprivileged" is automatically added
to the FEATURES variable.

X-Gentoo-Bug: 433453
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=433453