Bug#699369: lintian: add a check for known non-free files in source packages by md5sum

[Stuart Prescott <stuart@debian.org>]

Package: lintian
Version: 2.5.10.3
Severity: wishlist

Looking at #657281 led me to look at Contents-source for other possible
occurrences of this file. I ended up using md5sum to positively identify that
the offending file in other source packages was or was not the same file.

That this file was in 5 different source packages indicates that it is a fairly
popular move to copy this file into your project and then use it. If I could
automate testing for this file with md5sum, then lintian could automate keeping
more copies of it out of the archive in the future; it would be possible to
include a list of known-non-free files and emit an error when found.

Something like:

1d3fda2edb4a89ab60a23c5f7c7d81dd sRGB.icm is non-free; distribution of modified versions not permitted. See #657281.

Perhaps this would be too hard to maintain or there would be sufficiently few
examples of places where it could help... just a thought.

cheers
Stuart