Add kill signal logic to exec

[samtstern]

Description

Fixes #2477

Scenarios Tested

$ firebase emulators:exec "cat"
i  emulators: Starting emulators: firestore
⚠  firestore: Did not find a Cloud Firestore rules file specified in a firebase.json config file.
⚠  firestore: The emulator will default to allowing all reads and writes. Learn more about this option: https://firebase.google.com/docs/emulator-suite/install_and_configure#security_rules_configuration.
i  firestore: Firestore Emulator logging to firestore-debug.log
^C 
i  emulators: Received SIGINT (Ctrl-C) for the first time. Starting a clean shutdown.
i  emulators: Please wait for a clean shutdown or send the SIGINT (Ctrl-C) signal again to stop right now.
i  emulators: Shutting down emulators.
i  hub: Stopping emulator hub
i  firestore: Stopping Firestore Emulator

Sample Commands

N/A