Reinstate gconv-modules as the default configuration file

Reinstate gconv-modules as the main file so that the configuration
files in gconv-modules.d/ become add-on configuration.  With this, the
effective user visible change is that GCONV_PATH can now have
supplementary configuration in GCONV_PATH/gconv-modules.d/ in addition
to the main GCONV_PATH/gconv-modules file.