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.
    
    (cherry picked from commit c80c9079c895f8e0285682a99fb3f91b892b8734)