readline: buffer data to only emit 'line' on '\n'

In "terminal: false" mode.

(And fire it multiple times if multiple lines arrive at once.)

This is necessary because the Windows telnet client sends every single
keystroke as it's typed.

See: http://stackoverflow.com/questions/9962197/node-js-readline-not-waiting-for-a-full-line-on-socket-connections

Closes #3059.