Original commit message from CVS:

* gst/gstplugin.c: (gst_plugin_finalize), (gst_plugin_load_file):
* gst/gstplugin.h:
* gst/gstregistry.c: (gst_registry_lookup_locked),
(gst_registry_scan_path_level):
* gst/gstregistryxml.c: (load_plugin):
Only ever load one plugin for a given plugin basename.
This ensures correct overriding of GST_PLUGIN_PATH over
GST_PLUGIN_SYSTEM_PATH and of home dir plugins over
system installed plugins.