post_setup_wheezy-fake-essential: ignore some purge failures

unconditional ucf or deluser usage won't be fixed for wheezy

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf: add flags-{start,end}-wheezy

add some fake-essential packages and ignore debsums errors in wheezy

Signed-off-by: Andreas Beckmann <anbe@debian.org>
add news entry about testing of partial suites and arbitrary upgrade paths