refactor(unit): Simplify dbus path code

The latest go-systemd changes two things:
1. use strings rather than objects for dbus paths
2. serialize special characters in dbus paths

This means we can drop the extra code to serialize paths
into dbus objects!
refactor(unit): unpack the response from dbus.New properly
refactor(unit): Use GetUnitProperties in stead of GetUnitInfo
refactor(unit): Call Subscribe manually
chore(logging): expand logging in SystemdManager start/stopUnit