(marker_byte_position): Renamed from marker_position.

(marker_byte_position): Renamed from marker_position.
(marker_position): New function returns the charpos.
(Fbuffer_has_markers_at): Test the marker's charpos.
(set_marker_restricted, Fset_marker): Set both kinds of position.
Optimize case where POSITION is a marker.
(set_marker_both, set_marker_restricted_both): New functions.
(Fmarker_position): Use the charpos.

(charpos_to_bytepos, bytepos_to_charpos): New functions.
(buf_charpos_to_bytepos, buf_bytepos_to_charpos): New functions.