Avoid code repetition in marker-related functions.

Avoid code repetition in marker-related functions.
* marker.c (attach_marker): New function.
(Fset_marker, set_marker_restricted, set_marker_both)
(set_marker_restricted_both): Use it.
(Fset_marker, set_marker_restricted, Fbuffer_has_markers_at):
Consistently rename charno to charpos.
(marker_position): Add eassert.
(marker_byte_position): Convert to eassert.