added full RelaxNG based config file syntax checking

added full RelaxNG based  config file syntax checking
which checks every filter according to it's own rules.
These are exported by the 'make' target to RelaxNG XML and XML Schema instances using the nifty 'trang' utility.
added check of all config files in etc dir according to both RelaxNG XML and XML Schema instances.
check targets are 'make check_xsd' and 'make check_rng' when standing in 'etc' directory.
Still missing to integrate XML checks into 'make check' target.