Nokia modems/handsets use their own protocol (phonet) for all messaging related
operations, currently unsupported, so don't bother trying to check messaging
support with the generic implementation.
Instead of the `_in_port()' variants, we'll provide more generic
`_full()' methods. These should be used either if we want to send commands
to a specific port, or if we want to use a specific user-provided GCancellable.
In addition to this user-provided GCancellable, we keep the internal modem-wide
one.
