epg: set modification time of all associated files to indexed

recordings to the stored timestamp and sync the rest to the main
recording file. When integrated with the likes of Jellyfin, a handful
of associated files are created for each recording, this makes all of
them have the same modification time. It is run on epg startup, so on
UNIX just by killing _epg, the modification times will be
synchronized. On Windows it'll be neccessary to close and open again
the proxy.

Signed-off-by: Javier Marcet <javier@marcet.info>