Fix int/EMACS_INT use in buffer.c.

Fix int/EMACS_INT use in buffer.c.

 buffer.c (struct sortvec): Use EMACS_INT for buffer positions.
 (struct sortstrlist, overlay_str_len): Use EMACS_INT for string
 length.
 (advance_to_char_boundary, Fset_buffer_multibyte)
 (overlays_at, overlays_in, mouse_face_overlay_overlaps)
 (overlay_touches_p, record_overlay_string, overlay_strings)
 (recenter_overlay_lists, fix_start_end_in_overlays)
 (modify_overlay, Fmove_overlay, report_overlay_modification)
 (evaporate_overlays): Use EMACS_INT for buffer positions.
 lisp.h (fix_start_end_in_overlays, overlay_touches_p): Adjust
 prototypes.