bug#15628: 24.3.50; ns-get-selection-internal quits on unsupported pasteboard content on OS X

[Sebastian Wiesner <lunaryorn@gmail.com>]

On OS X 10.8, "ns-get-selection-internal" calls quit on unsupported
pasteboard content, such as images.

This behaviour breaks all yanking and killing.  The quit signal
interrupts "kill-new" and "current-kill" while these functions try to
retrieve the pasteboard content via "ns-get-pasteboard" (called by
"x-selection-value", which is the default "interprogram-paste-function"
on OS X), which internally calls "ns-get-selection-internal".

Hence, "kill-new" fails to save the given kill to the kill ring, and
"current-kill" fails to pop the latest kill from the kill ring.

To reproduce, start Emacs, set "save-interprogram-paste-before-kill" to
t, copy an image (e.g. from an iPhoto Library), and then try to yank or
kill.  For the latter, the following stacktrace is given with
"debug-on-quit":

Debugger entered--Lisp error: (quit "empty or unsupported pasteboard type")
  ns-get-selection-internal(CLIPBOARD)
  ns-get-pasteboard()
  x-selection-value()
  kill-new(#("elisp-slime-nav" 0 15 (fontified t face font-lock-constant-face)))
  copy-region-as-kill(39166 39181)
  kill-ring-save(39166 39181)
  #<subr call-interactively>(kill-ring-save nil nil)
  ad-Advice-call-interactively(#<subr call-interactively> kill-ring-save nil nil)
  apply(ad-Advice-call-interactively #<subr call-interactively> (kill-ring-save nil nil))
  call-interactively(kill-ring-save nil nil)
  command-execute(kill-ring-save)

This unfortunate interaction makes "save-interprogram-paste-before-kill"
nearly useless on OS X, because the user always needs to inspect the
pasteboard manually and remove content unsupported by Emacs before being
able to reliably kill and yank.

I think "ns-get-selection-interal" should simply ignore unknown
pasteboard content, or at least respect a user option to ignore unknown
pasteboard content.  This unknown content is of no use to Emacs anyway,
so there is no point in even trying to put it onto the kill ring.