_slot_operator_update_probe: fix bug #528610

This fixes a case inside _slot_operator_update_probe where it would
select an inappropriate replacement_parent of a lower version than
desired. The problem is solved by rejecting replacement_parent if its
version is lower than the existing parent, and a downgrade is not
desired.

X-Gentoo-Bug: 528610
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=528610
Acked-by: Brian Dolbec <dolsen@gentoo.org>
unprivileged mode: use first_existing helper func

Split out a first_existing function, in order to improve logic related
to the _unprivileged_mode function so that it checks whether it's
possible to create the specified target root (instead of requiring that
the target root already exists).

Acked-by: Alexander Berntsen <bernalex@gentoo.org>