dissect-image: probe file system via main block device fd/image file fd

let's make sure we can probe file systems also when unprivileged:
instead of probing the partition block devices for file system
signatures, let's go via the original "whole" fd.

libblkid makes this easy actually, as it allows us to specify the
offset/size of the area to probe. And we have the partition
offsets/sizes anyway, so it's trivial for us to make use of.

This thus enables fs probing also when lacking privs and operating on
naked regular files without loopback devices or anything like this.

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