bug#15455: 24.3.50; wrong defcustom :type for `confirm-kill-emacs'

[Drew Adams]

The Value Menu in Customize wrongly give the user no way to specify an
arbitrary predicate, and wrongly gives the impression that the option
value must be one of the two predicates presented (or nil).

The :type spec should provide a third possibility for the menu: an
arbitrary function (which is used as a predicate).

That menu item's :tag should make clear that the function is used as
a predicate: nil means..., non-nil means....

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-09-20 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking 'CFLAGS=-O0 -g3' CPPFLAGS=-DGLYPH_DEBUG=1'