Currently, we determine NMD_CONNECTION_PROPS_EXTERNAL based
on the settings connection. That is not optimal, because whether
a connection is assumed or externally managed, should be really a
property of the active-connection. So, in the this will change soon
and we would need yet another argument to nm_dispatcher_call().

Instead, drop the settings-connection and applied-connection
arguments and fetch them from the device as needed (but allow
to pass a specific act-request argument to explicitly state
which active connection to use).

Also, rename nm_dispatcher_call() to nm_dispatcher_call_device(),
it this is not a generic dispatcher call, but it is particularly
related to device events. Likewise, rename nm_dispatcher_call_sync()
to nm_dispatcher_call_device_sync().