This may happen when sending commands to the modem while in non-enabled state,
like when sending the PIN. In this case, just send the command, don't fully
initialize the port with the initialization sequence.
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.
Users of mm_base_modem_at_command_in_port() can now directly pass the result
of mm_base_modem_peek_port_primary() or mm_base_modem_peek_port_secondary(),
which may be NULL if no such port exists in the modem. So we'll gracefully
detect the NULL port and complete the async operation with an error.
* 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 a tight connection between the action of sending AT commands, either single
or in a sequence, with the MMBaseModem object owning the port. This direct
relation allows sending commands without specifying which port to use, so that
the modem object can get the best port at each time, and handling all that in a
single common place.
The original mm-at API has also been modified so that when a single command is
sent, a constant string is returned. We are allowed to return constant strings
in mm_base_modem_at_command_finish() because the string itself is owned by the
GSimpleAsyncResult, and hence, alive enough time. The GSimpleAsyncResult is
completely disposed only after the async call is fully completed.
Same reasoning behind the GVariant returned in the AT sequences; it should not
be owned by the caller, it's a transfer-none in introspection terms.
