testnodes: create test-cert for mailman.example.org to avoid failing bootstrapping
hosting: Fix spamd restart bug
hosting: lock down exim depedencies on extra conffiles
dasz/defaults: configure stable-updates

See for https://wiki.debian.org/StableUpdates

This is good as those are packages that "many users may wish to install
on their systems before the next point release is made, such as updates
to virus scanners and timezone data."

This is not bad as "all packages from stable-updates will be included in
point releases."

This is required as stable-updates are used while creating the basebox and
may conflict with stuff installed later.