Fix demo build, various updates

* Fixed build for demos
  * Fixed out-of-tree-build in newsdb by adding `-I$(top_srcdir)/demo/newsdb` to ecppc call
  * Replaced `$(top_builddir)/tntnet-config` with `$(top_builddir)/misc/tntnet-config`
* Fixed silent rules in `demo/calci18n`, `demo/comp`
* Fixed a warning about the possibly uninitialized int id in `demo/newsdb`
* Renamed `utest` to `tntnet-test` (in compliance with `tntdb-test`)
* Added missing `@endcond internal` doxygen label
* Re-revised documentation of TntConfig