Add an optional ddxBeforeReset function that is called from dix/dispatch.c right
before the call to KillAllClients().

This function is required for building Cygwin's clipboard integration manager.

This change has been build and tested in production environments on Cygwin.

Note that ddxBeforeReset is defined in vfb, xnest, and Xprint at all times since
the DDX-specific build flag DDXBEFORERESET will get passed to DIX and it will
expect to be able to call ddxBeforeReset in these modules even though those
modules build flags did not include DDXBEFORERESET.  This is the standard manner
in which other such functions are defined in these modules.

Harold