set defaults for known-problem-directory

don't set known-problem-directory in piuparts.conf, there is
not really a need to change this from the default currently

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p: disable using pdiffs

pdiffs cost a lot of cpu in order to save some bandwidth
that's useless with a local mirror or proxy

Signed-off-by: Andreas Beckmann <anbe@debian.org>