Selection changes

[Chong Yidong]

I've updated the X selection code to provide support for clipboard
managers (plus some standards compliance bits like the MULTIPLE target).

It seems to work well with Gnome's minimalist clipboard manager; the
clipboard is no longer lost when deleting a frame or exiting Emacs.
Help with testing on other environments would be much appreciated, and
let me know if you experience any funky behavior.

Saving is done by the function x-clipboard-manager-save, run from
delete-frame-functions and kill-emacs-hook (though I'm considering
making this internal instead of using hooks).  The other ports should be
unaffected by these changes.