bug#58882: ido-everywhere & add-function

[Taylor Grinn <grinntaylor@gmail.com>]

I can modify `read-file-name-function` like so:

  (advice-add read-file-name-function :around #'my/setup-minibuffer)

However, with `ido-everywhere-mode` enabled, I have to do it like this:

  (advice-add 'read-file-name-function :around #'my/setup-minibuffer)

The reason is that ido-everywhere adds a function to the variable
'read-file-name-function, rather than the value of that variable. This
changes the type of 'read-file-name-function from a symbol to a
function. This seems like it might have been a mistake.

Changing all instances of `add-function', which is a macro, to
`advice-add', which is a defun, will allow the variables
'read-file-name-function and 'read-buffer-function and 'ffap-file-finder
to remain unchanged, which I think was the intended effect.

Best,

Taylor