systemctl: add add-wants and add-requires verbs

core: Verify systemd1 DBus method callers via polkit DBus methods that retrieve information can be called by anyone. DBus methods that modify state of units are verified via polkit action: org.freedesktop.systemd1.manage-units DBus methods that modify state of unit files are verified via polkit action: org.freedesktop.systemd1.manage-unit-files DBus methods that reload the entire daemon state are verified via polkit action: org.freedesktop.systemd1.reload-daemon DBus methods that modify job state are callable from the clients that started the job. root (ie: CAP_SYS_ADMIN) can continue to perform all calls, property access etc. There are several DBus methods that can only be called by root. Open up the dbus1 policy for the above methods. (Heavily modified by Lennart, making use of the new bus_verify_polkit_async() version that doesn't force us to always pass the original callback around. Also, interactive auhentication must be opt-in, not unconditional, hence I turned this off.)

core: Filter by state behind the D-Bus API, not in the systemctl client.

systemctl: add commands set-default and get-default systemctl set-default NAME links the default.target to the given unit, get-default prints out the path to the currently set default target.

move more main systemd parts to core/