Debian unstable is frozen for the Debian 9 release, current development
happens in experimental. After the release, this can be switched back to
master, and the branch set through the `$BRANCH` env variable in the
semaphore config.
This makes the default build much quicker. If people are building systemd for
packaging or actual installation, they probably need to set some more options
anyway (-Ddns-servers=, -Dntp-servers=), so adding -Dman=true is not a big
burden.
For CIs configured locally, -Dman=true is added to restore status quo ante.
