gst/avi/: Port from -THREADED to HEAD, and fix for latest API changes of the day. Keep avimux dead for now.

Original commit message from CVS:
* gst/avi/Makefile.am:
* gst/avi/gstavi.c: (plugin_init):
* gst/avi/gstavidemux.c: (gst_avi_demux_get_type),
(gst_avi_demux_class_init), (gst_avi_demux_init),
(gst_avi_demux_reset), (gst_avi_demux_index_next),
(gst_avi_demux_index_entry_for_time),
(gst_avi_demux_index_entry_for_byte),
(gst_avi_demux_index_entry_for_frame), (gst_avi_demux_src_convert),
(gst_avi_demux_handle_src_query), (gst_avi_demux_handle_src_event),
(gst_avi_demux_parse_file_header), (gst_avi_demux_stream_init),
(gst_avi_demux_parse_avih), (gst_avi_demux_parse_superindex),
(gst_avi_demux_parse_subindex), (gst_avi_demux_read_subindexes),
(gst_avi_demux_parse_stream), (gst_avi_demux_parse_odml),
(gst_avi_demux_parse_index), (gst_avi_demux_stream_index),
(gst_avi_demux_stream_scan), (gst_avi_demux_massage_index),
(gst_avi_demux_stream_header), (gst_avi_demux_handle_seek),
(gst_avi_demux_process_next_entry), (gst_avi_demux_stream_data),
(gst_avi_demux_loop), (gst_avi_demux_sink_activate),
(gst_avi_demux_sink_activate_pull):
* gst/avi/gstavidemux.h:
Port from -THREADED to HEAD, and fix for latest API changes of
the day. Keep avimux dead for now.