pre_install_exceptions: fail before mini-buildd-{bld,rep}/squeeze

mini-build-* in squeeze fails to install due to timeout

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/db: do not fail to hand out the last package

deleting someting from a list while iterating over it is a bad idea,
so return a copy of the list instead

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