lavf/segment: add stream_segment variant of the segment muxer

This simplifies usage for segment streaming formats with no global
headers, tipically MPEG 2 transport stream "ts" files.

The seg class duplication is required in order to avoid an infinite loop
in libavformat/utils.c:format_child_next_class().

libavformat/allformats.c

@@ -215,6 +215,7 @@ void av_register_all(void)
 #endif
     REGISTER_DEMUXER  (SEGAFILM, segafilm);
     REGISTER_MUXER    (SEGMENT, segment);
+    REGISTER_MUXER    (SEGMENT, stream_segment);
     REGISTER_DEMUXER  (SHORTEN, shorten);
     REGISTER_DEMUXER  (SIFF, siff);
     REGISTER_DEMUXER  (SMACKER, smacker);