* src/xdisp.c (syms_of_xdisp): New vars redisplay--all-windows-cause and

* src/xdisp.c (syms_of_xdisp): New vars redisplay--all-windows-cause and
redisplay--mode-lines-cause.
(redisplay_internal): Keep them uptodate.  Remove redundant check of
buffer_shared_and_changed.
* *.[chm]: Number every assignment to update_mode_lines so we
can track why it is set.