Include charset.h.

Include charset.h.
(gap_left, gap_right): Put an anchor `\0' at the end of GAP.
(adjust_markers_for_insert): If any markers are adjusted, check
and fix the order of overlays in overlays_before.
(make_gap): Allocate an extra byte and set to `\0' for anchoring.
(insert1, insert_from_buffer_1): Put an anchor `\0' at the head of
GAP.