base-modem: implement new modem-wide cancellable, and rework AT commands

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.

src/mm-base-modem.c

@@ -387,6 +387,22 @@ mm_base_modem_get_valid (MMBaseModem *self)
     return self->priv->valid;
 }
 
+GCancellable *
+mm_base_modem_peek_cancellable (MMBaseModem *self)
+{
+    g_return_val_if_fail (MM_IS_BASE_MODEM (self), NULL);
+
+    return self->priv->cancellable;
+}
+
+GCancellable *
+mm_base_modem_get_cancellable  (MMBaseModem *self)
+{
+    g_return_val_if_fail (MM_IS_BASE_MODEM (self), NULL);
+
+    return g_object_ref (self->priv->cancellable);
+}
+
 MMAtSerialPort *
 mm_base_modem_get_port_primary (MMBaseModem *self)
 {