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: live-replay output sequences lasting longer than the recording
   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.
reverts superstitious guard around sock.recv in proxy_to_coordinator

I had a bug that required a try/except guard around the sock.recv()
call in proxy_to_coordinator. Such guard is no longer needed.

This commit should be rebased into the main one before merging.
The only reason I haven't done it yet is because it would clobber
a discussion in a line note in the github pull request.

However, once you say everything is ok, I'll just push -f a flattened
version of the branch for a cleaner history when you merge.