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.
 * known bug: when typing, sometimes there is a bounce and double characters
   get inserted.
 * known bug: log output sequences get buffered waiting for the next
   user action.
 * known bug: when live-replaying appended logs, pias crashes out as it
   tries to play an eventlog past an exit in the pty
 * could use some testing with older eventlog recordings.
 * could use some testing with multi-terminal live replays.