Provide fping addresses via pipe, instead of an argument list. #exec_arguments defaults to a no-op, but #exec_input does not. This means that exec_callbacks modules that do no use #exec_input need to define a no-op themselves, as the default will still try to write data to stdin. (This may be addressed in the monitor at some point, it is a bit of a gotcha.) Rather than define an empty #exec_input method, make a functional one, and remove #exec_arguments. This will fare better if the input list is excessively long anyway, and it fixes Error::EPIPE.