Makefile: overhaul, install targets depend on build-stamp

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/read_config.sh: add some comments

describe the sed expression a bit

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf: set known-problem-directory

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