was only used once

It is only relevant there.

There were some locations referenceing a variable that was only
availabe with NXAGENT_ONSTART set

There was only one (commented) section using it.

This better reflects its purpose: Tell listeners we are ready.

They belong there...

Make them defines. Also determine the screen depth dynamically.

We either use a solid black or a white background and no backround
pixmap. So nxagentRootTileWindow is always empty and we can drop all
stuff around it.

remove nxagentSplashCount, too, since it is no longer checked anywhere.

nobody was checking it.

The splash window is only shown in x2go mode. In nxagent mode the
splash window was also shown, but empty (and thus invisible). And the
code waited for the splash window to disappear. Fix this by skipping
_all_ the splash stuff in nxagent mode.

Before there was no way of getting a white background despite having
the approriate code.

Backport of this commit:

  commit cb0a565d2b2cf8823abbd77b4426cc2237731dc1
  Author: Daniel Stone <daniel@fooishbar.org>
  Date:   Fri Aug 18 17:04:48 2006 +0300

    dix: add whiteroot flag
    Add a -wr option to use a white root window, and use a BackPixel rather
    than BackPixmap for both white and black root windows.

Fixes ArcticaProject/nx-libs#832

Fixes ArcticaProject/nx-libs#733

no more need to pass down a struct, we now only pass the milliseconds
and let the function do the rest.

The effect of this was that special keystrokes where detected and
passed to the nxagent. E.g. pressing ctrl-alt-f for Fullscreen also
produced an "f" in the current input window inside the nxagent.