add a callback to determine the starting window.

Our version only adds some commented code, so it is not really
necessary. But it is cleaner to handle it that way.

Attributes GH PR #851: https://github.com/ArcticaProject/nx-libs/pull/851

Backport of this commit:

  commit 7ba7085b4f01f3cd72008712a5333ea3f0edfd88
  Author: walter harms <wharms@bfs.de>
  Date:   Sat Oct 28 19:14:22 2017 +0200

    AuRead.c: remove redundant null check on calling free()

    this removes simply unneeded code from XauReadAuth
Signed-off-by: Walter Harms <wharms@bfs.de>
Reviewed-by: Daniel Martin <consume.noise@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

libXau is integrated into libNX_X11, but the differences to libXau
original code where not explicitly marked. This path adds some ifdefs
and also restores the original code.

Attributes to ArcticaProject/nx-libs#850

Attributes GH PR #845: https://github.com/ArcticaProject/nx-libs/pull/845

nxagent links system libraries that link against libX11. Unfortunately,
nxagent (and libXcompshad) require a modified libX11 version,
nicknamed libNX_X11, for proper functioning. Fortunately, this one can
act as a drop-in replacement for the system libX11. So we’ll hack our
way out: add a DT_NEEDED entry for libX11 by linking against the
system library when building nxagent and link the other system
libraries later and set DT_RUNPATH to a special directory containing
symlinks from libNX_X11 to libX11.

This tricks the loader into pulling in the "fake" libX11 version
without checking its SONAME and thus satisfying the DT_NEEDED entry
early on - specifically also for the system libraries.

Ex.:
readelf -a /usr/bin/nxagent | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libX11.so.6]
...
 0x0000000000000001 (NEEDED)             Shared library: [libNX_X11.so.6]
...

If, however, you run the build a second time, it will find the newly
created libX11 links in ../../exports/lib (used as an additional
library search path) instead of the system libX11. The results in a
binary looking like this:

 0x0000000000000001 (NEEDED)             Shared library: [libNX_X11.so.6]
...

With such a setup, the whole magic falls apart and system libraries
will suddenly pull in the actual system libX11 file.

We initially believed a compiler call such as:

${CC} [--unrelated-options ...] [-Lunrelated_library_path ...] -lX11 [more
       options like -l and -L] -L../../exports/lib -lother_libraries

to do "the right thing" and link against the system libX11 (or, at
worst, a libX11 found in the library directory search list preceding the
link call) and use ../../exports/lib only for later linking operations,
since the order of options matters.

However, this turned out to be blatantly wrong: while the order of -L
arguments does matter for building the search path, the whole search
path including elements from *ALL* -L arguments (and the system paths)
will always be used when linking libraries.

Hence, (counter-intuitively to us) both these calls will be equivalent:

${CC} [--unrelated-options ...] -la -Loverride_liba -lb
      [-Lunrelated_library_path ...] -lc

${CC} [--unrelated-options ...] -Loverride_liba [-Lunrelated_library_path
       ...] -la -lb -lc

By removing LDPRELIBS from NXAGENTSYSLIBS, ../../exports/lib is no
longer used during building/linking and the binaries are built
reproducibly. We never intended to use this directory at link time
anyway.

Attributes GH PR #849: https://github.com/ArcticaProject/nx-libs/pull/849

Remove some code that is no longer needed because the code requiring
it has been removed some time ago (commit
643e13bf). Fixes a small memleak that
turned up after switching to fullscreen once.

Attributes GH PR #848: https://github.com/ArcticaProject/nx-libs/pull/848

Fixes: ArcticaProject/nx-libs#847

Using arguments that need a parameter without providing one would lead
a segfault due to calling strcmp() will NULL. Triggered by running
"nxagent -ac :1 -options nx/nx,fullscreen" or similar,

Fixes ArcticaProject/nx-libs#847

Attributes GH PR #846: https://github.com/ArcticaProject/nx-libs/pull/846

Use a more descriptive name.

Fixes ArcticaProject/nx-libs#632

This avoids delays, timeouts and missing copy/paste actions.

add new debug output, extend existing, drop meaningless

serverClientCutProperty is the external equivalent of the internal
clientCutProperty. We need it on the server side, too, because we use
the property on the serverWindow on the real X server. We could
(mis)use serverCutProperty here but this might introduce race
conditions when both sides request selections simultaneously.