Xserver/Xext/shmint.h: Provide header file exporting ShmRegisterFunc and ShmRegisterFbFuncs (plus ShmSetPixmapFormat as a by-product).

 commit 9ceffb6b92e55f0d74d71489900d43940f14dfd9
 Author: Eric Anholt <anholt@freebsd.org>
 Date:   Wed Dec 28 10:37:17 2005 +0000

    Merge from xserver a header for shm's server internal functions, which are
        called by DDXes. Desired by XGL, and should be used in other locations,
        probably.
Backported-to-NX-by: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>