Selection changes

[Chong Yidong]

I've changed the way Emacs interacts with the X clipboard and selection,
to bring it in line with how other X applications behave.  For most
users, the main visible change is that `kill' and `yank' now interact
with the clipboard.  One advantage of this change is that the
Cut/Copy/Paste menu bar items are just the usual C-w/M-w/C-y commands.

I believe that this change should be pretty much seamless, but let me
know if there is any problems.