Refactor the Unsquashfs -help text

[plougher]

The list of options in Unsquashfs has got quite large over the last couple of releases. This can make people not find the option they want or simply glaze over.

Try to refactor the help text into separate functional aspects, as was done a couple of years back for Mksquashfs.

Having done so, it may be possible to add some more options - without making it even more of a random list of options.