Instead of saving the old link colours when selecting a link and using that
to restore them when unselecting it, just copy the data from the document.
- Eliminate struct link_bg and the .link_bg and .link_bg_n members
of struct document_view.
- Eliminate the free_link routine and don't call it from draw_doc,
clear_link, or detach_formatted.
- Add a .old_current_link member to struct view_state and initialise it in
init_vs.
- Don't save link_bg in draw_current_link.
- Rewrite clear_link to use the document data instead of link_bg.
- Modify init_link_drawing not to allocate ling_bg and to return a pointer
to a static variable for the template character.
