iconvdata: Move gconv-modules configuration to gconv-modules.conf

Move all gconv-modules configuration files to gconv-modules.conf.
That is, the S390 extensions now become gconv-modules-s390.conf.  Move
both configuration files into gconv-modules.d.

Now GCONV_PATH/gconv-modules is read only for backward compatibility
for third-party gconv modules directories.

Reviewed-by: DJ Delorie <dj@redhat.com>