unprivileged mode: generate PORTAGE_DEPCACHEDIR

For unprivileged mode, if PORTAGE_DEPCACHEDIR is unset and the default
PORTAGE_DEPCACHEDIR setting does not refer to a writable directory
(or there are not sufficient permissions to create it), then
automatically make PORTAGE_DEPCACHEDIR relative to the current target
root (which should always be writable for unprivileged mode). Also, in
create_trees, propagate the automatically generated PORTAGE_DEPCACHEDIR
setting to the config instance that is instantiated for ROOT = "/".

The result is that unprivileged mode will get a writable
PORTAGE_DEPCACHEDIR by default, and the default can be overridden by
setting the PORTAGE_DEPCACHEDIR variable.

Fixes: 1364fcd89384 ("Support unprivileged mode for bug #433453.")
X-Gentoo-Bug: 433453
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=433453