Create reproducible non unique files when muxing or encoding

Also prevents adding Unique ID and ffmpeg/libav version to files.

'-fflags +bitexact' affects only muxer and '-flags +bitexact' affects only encoder:

https://libav.org/avconv.html
https://git.libav.org/?p=libav.git;a=blob;f=tests/fate-run.sh;h=a0a69ad0a7ca716e008446eea67f69b19fb14840;hb=HEAD#l122

https://www.ffmpeg.org/ffmpeg-formats.html#toc-Format-Options
https://www.ffmpeg.org/ffmpeg-codecs.html#toc-Codec-Options
https://git.videolan.org/?p=ffmpeg.git;a=blob;f=tests/fate-run.sh;h=1a8ff940bd92f4e301435c49d0dc9331bda61700;hb=HEAD#l143

youtube_dl/downloader/hls.py

@@ -30,7 +30,7 @@ class HlsFD(FileDownloader):
 
         args = [
             encodeArgument(opt)
-            for opt in (ffpp.executable, '-y', '-i', url, '-f', 'mp4', '-c', 'copy', '-bsf:a', 'aac_adtstoasc')]
+            for opt in (ffpp.executable, '-y', '-i', url, '-f', 'mp4', '-c', 'copy', '-bsf:a', 'aac_adtstoasc', '-fflags', '+bitexact', '-flags', '+bitexact')]
         args.append(encodeFilename(tmpfilename, True))
 
         retval = subprocess.call(args)

youtube_dl/postprocessor/ffmpeg.py

@@ -131,6 +131,7 @@ class FFmpegPostProcessor(PostProcessor):
             os.stat(encodeFilename(path)).st_mtime for path in input_paths)
 
         files_cmd = []
+        opts += ['-fflags', '+bitexact', '-flags', '+bitexact']
         for path in input_paths:
             files_cmd.extend([encodeArgument('-i'), encodeFilename(path, True)])
         cmd = ([encodeFilename(self.executable, True), encodeArgument('-y')] +