Correct the writing of special case options to the config file
Define strings to store special option parsing parameters leaving no OPT_WITH_ARG missing args
Provide a ccan variant OPT_WITH_CBARG that assigns the arguments passed as a string and then performs the callback function on the string.
Use OPT_WITH_CBARG for all custom parsing functions to allow their values to be written generically when writing the config file from the menu.