Exit request support (shm?)

[grandrew]

Add exit request support for external helper to be able to control daemons' states

• support exit while connected
• support exit at client side while attempting to connect
• support exit for server process
• fd server exit request support (check SHM)

server and client connecting processes can receive ordinary signals and exit; but it can be a challenge to get their PID

• client may update their SHM status pid so that helper can find them; but as their process_num is unkown at this time it is inconsistent to use the available fields;
• connecting client may check SHM field that indicates exit request, at process_num 0
• server listening process may check exit request at SHM index 0 process_num 0; should receive a SIGHUP in order to exit though as it waits on accept indefinitely; need to check that connected processes safely ignore it