💥 revise hook/var fns naming convention (2/2)

This is second of three big naming convention changes. In this commit,
we change the naming conventions for hook functions and variable
functions:

1. Replace the bar | to indicate a hook function with a -h suffix, e.g.

     doom|init-ui -> doom-init-ui-h
     doom|run-local-var-hooks -> doom-run-local-var-hooks-h

2. And add a -fn suffix for functions meant to be set on variables,
   e.g.

     (setq magit-display-buffer-function #'+magit-display-buffer-fn)

See ccf327f8 for the reasoning behind these changes.

modules/ui/doom-quit/config.el

@@ -22,11 +22,11 @@
   "A list of quit messages, picked randomly by `+doom-quit'. Taken from
 http://doom.wikia.com/wiki/Quit_messages and elsewhere.")
 
-(defun +doom-quit (&rest _)
+(defun +doom-quit-fn (&rest _)
   (doom-quit-p
    (format "%s  Quit?"
            (nth (random (length +doom-quit-messages))
                 +doom-quit-messages))))
 
 ;;
-(setq confirm-kill-emacs #'+doom-quit)
+(setq confirm-kill-emacs #'+doom-quit-fn)