bug#17745: 24.3; auto-mode-alist .ad for conf-xdefaults-mode

[Kevin Ryde <user42_kevin@yahoo.com.au>]

files.el auto-mode-alist has an entry

     ("/X11.+app-defaults/" . conf-xdefaults-mode)

It could be good if it also matched Foo.ad files, like

     ("/X11.+app-defaults/\\|\\.ad\\'" . conf-xdefaults-mode)

"Foo.ad" is the name of an app-defaults file in a source code
distribution.  It's renamed to just "Foo" when installed to
/etc/X11/app-defaults/.

(For comparison i can be seen generic-x.el `x-resource-generic-mode' has
.ad treated this way.)

2014-06-10  Kevin Ryde  <user42_kevin@yahoo.com.au>

	* files.el (auto-mode-alist): .ad files as conf-xdefaults-mode.




In GNU Emacs 24.3.1 (i486-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2014-05-06 on biber, modified by Debian
Configured using:
 `configure '--build' 'i486-linux-gnu' '--build' 'i486-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var/lib' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 '--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3/site-lisp:/usr/share/emacs/site-lisp'
 '--with-crt-dir=/usr/lib/i386-linux-gnu' '--with-x=yes'
 '--with-x-toolkit=lucid' '--with-toolkit-scroll-bars' '--without-gconf'
 '--without-gsettings' 'build_alias=i486-linux-gnu' 'CFLAGS=-g -O2
 -fstack-protector --param=ssp-buffer-size=4 -Wformat
 -Werror=format-security -Wall' 'LDFLAGS=-Wl,-z,relro'
 'CPPFLAGS=-D_FORTIFY_SOURCE=2''