Hi Guix,

This patch adds MAT, the Metadata Anonymisation Toolkit from Boum.  I've
successfully used its CLI tool to purge metadata from JPEG image files;
I verified using exiftool that it works for this purpose.  However, not
all of its features work (see the TODO for details), and more
importantly, the website says people shouldn't use it.  For these
reasons, I'm not sure if we should add it, so I'd like to ask for your
opinion.

The author state on their website:

  https://mat.boum.org/

  Current status

  The MAT maintenance and development is currently on hold, mostly for
  health reasons. I might go back to it at some point in the future.

  The current version might have bugs, and doesn't work on Python3: Please
  avoid using it.

However, packages exist for some distributions.  For example, here's a
MAT package for Debian:

  https://packages.qa.debian.org/m/mat.html

And like I said, the CLI tool does seem to work.

Should we refrain from adding this package simply because the author is
not maintaining it any more?  I'm inclined to say "no", but one also has
to consider whether it is a a good idea to encourage people to use an
unmaintained tool for protecting their privacy/anonymity.  I'm not sure.

In addition, I notice that the license is GPL 2, but it seems the author
did not specify whether "any later version" can be used.  Therefore, I
have listed this as gpl2, instead of gpl2+.

What do you think?

-- 
Chris