(Vfringe_bitmaps): New variable.

(Vfringe_bitmaps): New variable.
(syms_of_fringe): DEFVAR_LISP it.
(valid_fringe_bitmap_p): Rename from valid_fringe_bitmap_id_p.
Change arg to Lisp_Object and fail if not an integer.
(get_fringe_bitmap_name, resolve_fringe_bitmap)
(destroy_fringe_bitmap): New functions.
(Fdestroy_fringe_bitmap): Change arg to bitmap symbol.  Use
destroy_fringe_bitmap.  Remove symbol from Vfringe_bitmaps and
clear its fringe property.
(init_fringe_bitmap): Use destroy_fringe_bitmap instead of
Fdestroy_fringe_bitmap.
(Fdefine_fringe_bitmap): Add BITMAP arg specifying new or existing
bitmap symbol; remove WHICH arg.  Add symbol to Vfringe_bitmaps
and set fringe property.  Signal error if no free slots.
(Fset_fringe_bitmap_face): Change arg to bitmap symbol.
(Ffringe_bitmaps_at_pos): Return bitmap symbols instead of numbers.