iconvconfig: Read configuration from gconv-modules.d subdirectory
In addition to GCONV_PATH/gconv-modules, also read module
configuration from *.conf files in GCONV_PATH/gconv-modules.d. This
allows a single gconv directory to have multiple sets of gconv modules
but at the same time, a single modules cache.
With this feature, one could separate the glibc supported gconv
modules into a minimal essential set (ISO-8859-*, UTF, etc.) from the
remaining modules. In future, these could be further segregated into
langpack-associated sets with their own
gconv-modules.d/someconfig.conf.