Fixes ArcticaProject/nx-libs#546.

  o Install nxagent/nxproxy executables into multi-arch path.
  o Fake libNX_X11.so* as $LIBDIR/nx-X11/libX11.so*.
  o Allow injecting different $NXAPP containing the executable name.

  o For e.g. x2goagent this requires the following changes:

    - Move x2goagent symlink from /usr/lib/x2go/bin/x2goagent to
      $LIBDIR/nx/bin/x2goagent
    - Replace /usr/bin/x2goagent by a two-liner...

    ```
    #!/bin/sh

    export NXAPP=x2goagent
    nxagent
    ```

 The x2goagent package is being replaced by an x2goserver bin:package
 named x2goserver-x2goagent.

 This x2goserver-x2goagent bin:package will be maintained in the X2Go
 Project's namespace and basically wraps around nxagent.

 Furthermore, declare Arctica Project as upstream source for nx-libs.

  * Debian/Ubuntu packaging:
    + Fully rework the way nx-libs gets packaged for Debian/Ubuntu.
    + Split up libnx-x11 into individual packages.
    + Provide dbg:packages for each bin:package containing binaries.
    + Use Makefile logic to install files into DESTDIR.
    + Provide dev:packages for each lib:package individually.
    + Provide nx-x11proto-*-dev packages for all libnx-* libraries.
    + Install _all_ library files (*.so*) to /usr/lib/<triplet>/, so
      no extra settings of LD_LIBRARY_PATH is necessary.

Drop patch 032, fix other patches so that installation via make install installs NX components to /usr/local/lib/nx and /usr/local/include/nx.

  - 005_add-nxagent-wrapper.patch.
  - 008_add-nxproxy-wrapper.patch.
  - 011_add-nxauth-wrapper.patch.