Mount over passwd and related files from host

Mount over passwd and related files from host

Package managers have the annoying quirk that they use the user
and group information from the host system instead of the user and
group information from the chroot. As a workaround, in
run_with_apivfs(), let's overmount the files from the host with the
ones from the root if they exist.

Let's also always install a distribution in 2 steps. First, we install
the package that provides passwd. Then, we install the rest of the
packages, during which passwd and related files will be overmounted.