libctrl: Add support for 'deferred control commands'

Sometimes a control interface command cannot be processed
and responded immediately, but we need to process it asynchronously.

In order to support this, we introduce the 'ctrl_cmd_def', which
represents such a deferred command.  It is created by the service
implementing the command using ctrl_cmd_def_make(), and a response is
later sent using ctrl_cmd_def_send().

ctrl_cmd_def_is_zombie() must be called to handle the case where
the control connection has disconnected/died between receiving the
command and sending the response.

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