The check-patch-files script checks that all patch files in the
given directory are annotated with an 'Upstream-Status', and
that all patch files are at least noted in their top-level
directory (so we can assume they are used).

In case a violation is detected, it returns a non-zero exit code,
so that a travis-ci reports a violation as build failure.

This addresses issue #39.

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>