add --live-replay option

 * --live-replay switch is ignored when a terminal is joining
 * live replay tries to use the shell that was used for recording
 * recorder now registers the shell used for recording in the eventlog
 * older eventlogs have no shell property, so replay uses default shell
 * --shell option allows to override the shell selection for live_replay
 * could use some testing with older eventlog recordings
 * could use some testing with multi-terminal live replays