avformat/hlsenc: add single file mode

HLS version 4 offers the possibility to keep the media file whole instead of
splitting it. In that case, segments are specified with byte ranges.

We introduce a new '-hls_flags' option for the hlsenc muxer, with a single
flag for the time being, 'single_file'.
avformat/hlsenc: add a flag disabling the filename in segment names

The default way to name segment is to use the name of the media file, with an
optional prefix (-hls_base_url).

However in applications where the URL used to fetch segments does not contain
the name of the file, it is useful to have the ability not to include the
filename in segment names.

Of course this only makes sense in single file mode, since without it,
segments are stored in individual files.
avformat/hlsenc: add an option to set the media filename

Works with or without the 'single_file' HLS flag.