bug#23179: 25.0.92; Restore `M-,' to continue etags search

[Dmitry Gutov]

On 04/06/2016 03:55 AM, John Wiegley wrote:
> there still needs to be a third
> choice, for specifying an arbitrary function.

Sure, and we can also use `radio' and `function-item' like in 
message-send-mail-function's definition. My sole point is that a 
defcustom where the value is a function is a reasonable, and in this 
case a desirable, thing to do.

Maybe we shouldn't advertise the "write your own function" possibility 
too much, since the required signature can still change abruptly, but 
that's a minor concern.