Needs to be an XlibAtom!

Fixes ArcticaProject/nx-libs#165

  commit dfd682b582636a36345144bcf835e3ee46718d90
  Author: Alan Coopersmith <alan.coopersmith@sun.com>
  Date:   Wed Jan 2 19:27:22 2008 -0800

    X.Org bug 4947/Sun bug 6646626: Xv extension not byte-swapping properly

    X.Org Bugzilla #4947 <https://bugs.freedesktop.org/show_bug.cgi?id=4947>
    Sun bug 6646626 <http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6646626>

    Don't use swapped data after swapping it.   When done swapping data,
    send the swapped data, not the address of the pointer to it, to the client.

Extend nxagentHandleClientMessageEvent: better debug messages,
suppress warning "Invalid window in ClientMessage" for legal case.

I came across this while testing some clipboard stuff.

had been forgotten in 7a2836f8

minimize differences to dix/dispatch.c

Backport of this xorg-xserver upstream patch:

  commit 20c2a3bcb11b5baf564e2c73a477ba23f5ae2b10
  Author: Peter Harris <pharris@opentext.com>
  Date:   Mon Jul 15 19:44:29 2013 -0400

    mi: Avoid stack smash when drawing dashed lines

    X.org Bug 54013 <https://bugs.freedesktop.org/show_bug.cgi?id=54013>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Peter Harris <pharris@opentext.com>

 Relates to ArcticaProject/nx-libs#890.

 Regression fix for 3cd622cf.

 Fixes ArcticaProject/nx-libs#890.

Attributes GH PR #877: https://github.com/ArcticaProject/nx-libs/pull/877

Unfortunately we cannot rename NX_CUT_BUFFER_SERVER, too, without
breaking compatibility because this one is used to signal nomachine's
nxclient after the splash screen is gone (see Splash.c, Window.c and
ArticaProject/nx-libs#838)

the MakeAtom call previously used is as good as using a variable but
with the variable it is a) easier to read/understand and b) consistent
with the other client* variables.

Using this macro makes the code more readable