Support unprivileged mode for bug #433453.

This takes the existing unprivileged prefix support and enables it to
work when EPREFIX is empty. This "unprivileged" mode 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 doebuild.py 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