gst-libs/gst/riff/: Add gst_riff_init() to initialize the debug category, instead of plugin_init(). Port riff-media.[...

Original commit message from CVS:
* gst-libs/gst/riff/riff-media.c: (gst_riff_create_video_caps),
(gst_riff_create_audio_caps), (gst_riff_create_iavs_caps),
(gst_riff_create_video_template_caps),
(gst_riff_create_audio_template_caps),
(gst_riff_create_iavs_template_caps):
* gst-libs/gst/riff/riff-media.h:
* gst-libs/gst/riff/riff-read.h:
* gst-libs/gst/riff/riff.c: (gst_riff_init):
Add gst_riff_init() to initialize the debug category, instead
of plugin_init(). Port riff-media.[ch] from -THREADED to HEAD.