libavformat/hlsenc: Allow usage of 'periodic-rekey' with multi-variant streams

libavformat/hlsenc: Allow usage of 'periodic-rekey' with multi-variant streams

This patch adds possibility to use 'periodic-rekey' option with
multi-variant streams to hlsenc muxer. All streams variants
use parameters from the same key_info_file.

There are 2 sets of encryption options that kind of overlaps and add
complexity, so I tried to do the thing without changing too much code.

There is a little duplication of the key_file, key_uri, iv_string, etc
in the VariantStream since we copy it from hls to each variant stream,
but generally all the code remains the same to minimise appearing
of unexpected bugs. Refactoring could be done as a separate patch then as needed.

Signed-off-by: Yaroslav Pogrebnyak <yyyaroslav@gmail.com>