[RFC] Allow block device providers to veto file systems

Hey all,

I've spent some time thinking about possible solutions for #414638 which all essentially worked around the fact that partman offers file systems (via valid_filesystems) that are not actually valid for certain crypto setups.

So I thought it would be useful to have a mechanism for "providers" of block devices to veto the use of certain file systems on the devices they provide, because they know that those choices won't work [1].

I've pondered different ways of implementing this, and ended up with the attached patch. There are two things I don't like about it: Since we are piping the list of filesystems through the veto scripts, any error in them can cause the list to end up empty. The scripts have to be extra careful not to consume stdin by accident.

The second thing I don't like but couldn't come up with anything better is the name 'valid_filesystems_veto'. If the basic idea is sound, and anyone has suggestions for a better name of the directory, I'm all ears :-)

        Max

-- 
[1] Otherwise we have to catch those "invalid" choices
in e.g. check.d or finish.d scripts, warn the user and
tell them to go back and fix it themselves. I feel we 
already have too many of those rather user-unfriendly 
checks in partman-crypto. If we can, we should IMO try
prevent invalid choices in the first place.

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