[3/4] include: Add twain 2.x header for use in data sources.

[Vincent Povirk]

> Doesn't the above essentially say we can't distribute this?

I'd say it gives no indication either way. The existing twain.h in our
source tree is the same.

Looks like this is the actual license: http://www.twain.org/twain-license/

Not sure if that lets us use it or not.

The official 2.x DSM is LGPL licensed and hosted at
https://github.com/twain/twain-dsm. It includes twain.h in its source
tree.

(How do I know that DSM is official? The twain.org website under
"Specification" links to a project on sourceforge that links to that.
And it's mentioned in the PDF spec, which says to obtain the Windows
DSM from twain.org. Is that sufficient documentation for our purposes?
I don't know.)