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.
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.
