8 v2 correct xsane et al snprintf under artistic

xsane, sane-frontends, sane-backends, and sane-backends-extras all use slightly different local snprintf s drawn from an old release of LPRng which are under the Artistic License. The autoconf setup isn't good enough to detect and use the system snprintf.

The problem was also reported to Fedora and as per spotrh's comment in User Issue 10713 they solved the problem by refactoring the more recent and acceptably licensed snprintf.c from current LPRng. Which they have sent to Sane. The xsane patch spotrh provided was used to reconstruct this and rather than manage four patches these four essentially identical helper commits copy this over the existing file and update debian/copyright with a shim. The shim is kept in DATA/blah with the new file for clarity of licensing of the code.