Convert more function definitions to standard C.

Convert more function definitions to standard C.
* src/xdisp.c (window_box_edges, handle_single_display_spec)
(display_string): Convert definition to standard C.
* src/scroll.c (do_direct_scrolling, scrolling_1):
* src/dispnew.c (allocate_matrices_for_frame_redisplay)
(mirrored_line_dance):
* src/coding.c (code_convert_string):
* src/charset.c (map_charset_chars):
* src/ccl.c (Fccl_program_p, Fccl_execute, Fccl_execute_on_string)
(Fregister_ccl_program, Fregister_code_conversion_map):
* src/keyboard.c (kbd_buffer_nr_stored): Likewise.
(head_table): Make static and const.