sd-bus: Remove bus arg from bus_verify_polkit_async_registry_free()

It's unneccessary, not used, and complicates callers of the
function.
core: Common code for DBus methods that Cancel a job

Both ofs.Job.Cancel() and ofs.Manager.CancelJob() now use same
implementation. So we can add caller verify logic appropriately.
core: Rename Job.subscribed field to Job.clients

This reflects how this field will be used, to not only track where
to send signals, but also which callers (other than root) are allowed
to call DBus methods on the Job.
Merge commit 'b39a2770ba55637da80e2e389222c59dbea73507'
sd-bus: add API to query which handler/callback is currently being dispatched
bus-util: simplify bus_verify_polkit_async() a bit

First, let's drop the "bus" argument, we can determine it from the
message anyway.

Secondly, determine the right callback/userdata pair automatically from
what is currently is being dispatched. This should simplify things a lot
for us, since it makes it unnecessary to pass pointers through the
original handlers through all functions when we process messages, which
might require authentication.
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.)
networkd: warn when ignoring unsupported tuntap options

The interface for creating tuntap devices should be ported to rtnl so it would support the same settings
as other kinds. In the meantime, the best one can do is to drop in a .link file to set the desired options.
Merge remote-tracking branch 'origin/master'