systemctl: rework how we connect to busses

systemctl: rework how we connect to busses

Sometimes we have to connect to the system manager directly (early boot,
initrd, late boot, ...), sometimes through the system bus (unprivileged,
remote, logind, ...). Instead of guessing in advance, which kind of
connection we require (and sometimes guessing incorrectly), let's make
sure each time we need  bus connection we request the right bus
explicitly.

This way, we set up exactly the bus connections require, never guess
incorrectly, and do so only immediately when necessary.

As effect this reworks avoid_bus() into install_client_side(), since
that's all it determines now: whether to install unit files client-side
or server-side (i.e. in PID 1).