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-sms.c

@@ -568,7 +568,6 @@ sms_store (MMSms *self,
                               cmd,
                               10,
                               FALSE,
-                              NULL, /* cancellable */
                               (GAsyncReadyCallback) store_ready,
                               ctx);
     g_free (cmd);
@@ -645,7 +644,6 @@ sms_send_generic (SmsSendContext *ctx)
                               cmd,
                               10,
                               FALSE,
-                              NULL, /* cancellable */
                               (GAsyncReadyCallback)send_generic_ready,
                               ctx);
     g_free (cmd);
@@ -683,7 +681,6 @@ sms_send_from_storage (SmsSendContext *ctx)
                               cmd,
                               10,
                               FALSE,
-                              NULL, /* cancellable */
                               (GAsyncReadyCallback)send_from_storage_ready,
                               ctx);
     g_free (cmd);
@@ -814,7 +811,6 @@ delete_next_part (SmsDeletePartsContext *ctx)
                               cmd,
                               10,
                               FALSE,
-                              NULL, /* cancellable */
                               (GAsyncReadyCallback)delete_part_ready,
                               ctx);
     g_free (cmd);