pmb.config: Check for kpartx and losetup with --json support

Running these inside the chroot is tricky, and we ended up reverting previous commits that attempted to make them run there (see 1ec55fc1). As such, just accept defeat and ensure that the host has these.

Also see #2465 (closed)

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Additionally, some other minor fixes. Please test this on a system without kpartx and losetup from util-linux. I tested it with the CI, and it seems to work as expected.

To test this, run $ pmbootstrap init and ensure that it errors out of losetup and kpartx aren't installed. Additionally, it should error if losetup from BusyBox is used.