refactor(options)!: graduate shortmess+=f flag

Not everything needs to be crazy overconfigurable.

Also fixes a warning in latest clang which didn't approve of
the funky math switch statement in  append_arg_number

src/nvim/options.lua

@@ -7247,7 +7247,6 @@ return {
         messages, for example  with CTRL-G, and to avoid some other messages.
         It is a list of flags:
          flag	meaning when present	~
-          f	use "(3 of 5)" instead of "(file 3 of 5)"		*shm-f*
           i	use "[noeol]" instead of "[Incomplete last line]"	*shm-i*
           l	use "999L, 888B" instead of "999 lines, 888 bytes"	*shm-l*
           m	use "[+]" instead of "[Modified]"			*shm-m*