Properly handle suspend from terminal (SIGTSTP) to support resume (SIGCONT)

[zalberico]

Summary: Currently when you suspend the urbit process from the terminal (via control-z) it gets treated as if it was sent a SIGTERM, the urbit is killed and the process exits and cannot be resumed. The correct way to treat a SIGTSTP is to get the urbit process into a good state and then send ourselves a SIGSTOP which suspends it. The user is then dropped back to a standard unix prompt. Then when the user attempts to resume the process (SIGCONT) the user gets the previously suspended process back.

Why: Handling this allows a user to quickly drop into the unix prompt without needing to wait for shutdown, it also allows the user to quickly resume the same process. There are alternatives (using tmux, screen, etc.) but we should properly support the signals received (which is what the user should expect).

Links

• For details about these signals: https://stackoverflow.com/questions/11886812/what-is-the-difference-between-sigstop-and-sigtstp#11888074
• The old PR that handles SGTSTP as SIGTERM: Exit on SIGTSTP. #592
• The older issue where this was discussed: We don't handle suspend from terminal too well #621