make: Refine Makefile

  * Early work to prepare for packaged version.
package: Adapt configure and Makefiles

  * Support out-of-tree compilation
    * sysconfdir, prefix, srcdir, top_srcdir
  * Remove hard-coded makefile paths
doc: Improve documention of configuration files

  * Improve documentation of configuration files
conf: Add some more documentation on netplex conf.
package: Add support for Debian packaging.

  * This allows to generate two debian packages (server and clients).
doc: Document the new default behavior for LIBNAME

  * Document feature corresponding to commit 74d74e1a22
package: Fix lintian issues

  * Add a ChangeLog
  * Fix various Debian lintian issues
  * Remove some remaining debug symbols in release