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.

Nokia phones don't support AT+CRSM.

This speeds up probing, as we cancel other AT probings as soon as we grab the
first AT port.

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.

 * mm_base_modem_peek_port_* () will return either a port object (no new
reference), or NULL if none available.

You would usually peek() a port if you're going to use it just in the current
method, as there is no way to that reference to get invalid (we're single
threaded).

 * mm_base_modem_get_port_* () will return either NEW references to valid
port objects, or NULL if none available.

And, you would usually get() a port, whenever you want the port object to be
valid even out of the current method, for example when keeping it in the
context of an async operation.

Also, we need to consider that the primary AT port MAY BE NULL when you
peek() or get() it. This is due to the fact that we may be releasing ports
(due to device disconnection) in the middle of async operations.

Make it use a GVariant in the response processor, as the AT command handling in
the MMBaseModem.