Ensure session recordings are not duplicated

[spbnick]

Implement limiting recording to one per session.

Perhaps have a tlog-only-writable directory (tlog-rec will be SUID when #5 is fixed) with files named after session IDs, containing tlog-rec PIDs. Session IDs uniquely identify logins within a single system boot. Tlog-rec also uses them in its JSON messages.

So, if tlog-rec is started and finds that this session already has a file in
that directory, and the process with the stored PID is alive, it just drops the
privileges and execs the user shell.