debugger: also exit when the repl emits 'exit'

Exit the debug repl when repl emits 'exit'

Refs: https://github.com/nodejs/node-v0.x-archive/issues/5637
Fixes: https://github.com/nodejs/node-v0.x-archive/issues/5631
PR-URL: https://github.com/nodejs/node/pull/2369


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>