In order to implement the new error handling and debugging
mechanism for command handlers, we need to change the cmd_new()
callback to return a value.
This commit introduces cmd_new_ret(), which returns a value and
will be used only temporarily to handle the transition from
cmd_new().
That is, as soon as all command handlers are ported to cmd_new_ret(),
it will be renamed back to cmd_new() and the new error handling
and debugging mechanism will be added on top of it.
Signed-off-by: Luiz Capitulino<address@hidden>