Implement selective undo (by Paul Flinders).

Implement selective undo (by Paul Flinders).
(undo-copy-list, undo-copy-list-1): New functions.
(undo-make-selective-list, undo-delta): New functions.
(undo-elt-in-region, undo-elt-crosses-region): New functions.
(undo-adjusted-markers): New defvar.
(undo-start): New args BEG and END.
(undo): If arg or active region, pass args to undo-start.