💥 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.

bin/doom

@@ -77,7 +77,7 @@
         nil 'nomessage)
 
   (cond ((not noninteractive)
-         (doom|run-all-startup-hooks))
+         (doom-run-all-startup-hooks-h))
         ((and (not (cdr args))
               (member (car args) '("help" "h")))
          (usage))