avformat, hls: add a flag to signal unavailability of seeking

The seek function can just return an error if seeking is unavailable,
but often this is too late. Add a flag that signals that the stream is
unseekable, and use it in HLS.

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2018-01-xx - xxxxxxx - lavf 58.6.100 - avformat.h
+  Add AVFMTCTX_UNSEEKABLE (for HLS demuxer).
+
 2018-xx-xx - xxxxxxx - lavu 56.9.100 - aes_ctr.h
   Add method to set the 16-byte IV.