* 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.

Provide a default keystrokes.cfg in /etc/nx (for nxagent) and /etc/x2go (for x2goagent). (Fixes: #199).

Fix faulty symlink path. Make sure that x2goagent symlink does not point to absolute path somewhere within the build environment.

Main Makefile: Use absolute symlink when linking from $(X2GOLIBDIR) into $(NXLIBDIR) (i.e. linking x2goagent -> nxagent). (Fixes: #143).

Work-in patch from Jan Engehardt for working around different improper bash implementations of string-in-string replacing. (Fixes: #145).

Continue Reinhard's work on top-level install/uninstall. Esp. integrate debian/post-nx-build.sh into main Makefile. Make value of CONFIGURE variable traversable through the Makefile tree.

Instead, place the makefile into debian/Makefile.nx-libs. This makes the
makefile easier to edit.