sem: refine actor lifecycle

[knz]

Today the actor enters at the start of the play and leaves at the end.
It would be useful to tighten the relationship between actors and (a group of) processes by allowing an actor to only enter the stage later into the script, and leave before the end (possibly repeatedly).

To achieve this:

• each actor would have an "intrat" and "exit" action
• the compiler would compute stage presence and reject invalid interleaves
• command execution would record PIDs / PGIDs
• cleanup actions could enforce process kills