import-fs: make various options controllable via cmdline/env var

import-fs: make various options controllable via cmdline/env var

This basically does what the previous two commits did for systemd-import
+ systemd-pull but for systemd-import-fs.

This commit is a bit simpler though, as a --direct mode doesn't change
that much. It's mostly about not searching for existing, conflicting
images and not much else.