Simplify apivfs_cmd() and chroot_cmd()

Simplify apivfs_cmd() and chroot_cmd()

We move the setpgid logic to run(), avoiding the need to pass a tools
argument to chroot_cmd() and apivfs_cmd().

We also try to remove as much logic from these functions as possible.
Since we can't really assume that any logic we execute during the
function will still hold true in the sandbox, so it's best to delay
any logic execution until we're already in the sandbox (using the
--ro-bind-try options of bubblewrap).

We also rework the /etc/resolv.conf handling to simply make sure that
/run/systemd/resolve exists in the chroot since if /etc/resolv.conf
points to /run it'll almost certainly be to
/run/systemd/resolv/stub-resolv.conf.