Merge private/runtests into t/runtests

Now that we do not have to generate test locales and only have one
test runner, private/runtests is mostly obsolete.  So move the last
"interesting" bits from it into t/runtests and remove
private/runtests.

Signed-off-by: Niels Thykier <niels@thykier.net>
lintian: Require --include-dir and --user-dirs earlier

If given, these options should now be the first ones on the command
line (like lintian-info).

Signed-off-by: Niels Thykier <niels@thykier.net>