Summary: Write emulator PID file

Specify a pid file path, and FS-UAE Launcher/Arcade will write the PID
of the emulator to the specified PID file when the emulator starts, and
remove the PID file when the emulator exists.

This can be used to monitor the emulator process from the outside, and e.g.
create an external shutdown button (sending SIGTERM to the emulator process).
This can be useful for game cabinets.