Change algorithm used to adjust positions for undo in region

Change algorithm used to adjust positions for undo in region

* simple.el (undo-make-selective-list): New algorithm fixes
incorrectness of position adjustments when undoing in region.
(Bug#17235)
(undo-elt-crosses-region): Make obsolete.
(undo-adjust-elt, undo-adjust-beg-end, undo-adjust-pos): New
functions to adjust positions using undo-deltas.
* automated/undo-tests.el (undo-test-region-deletion): New test to
demonstrate bug#17235.
(undo-test-region-example): New test to verify example given in
comments for undo-make-selective-list.

Fixes: debbugs:17325