repl: create history file with mode 0600

Set the mode bits on the history file to 0o600 instead of leaving it
unspecified, which resulted in 0o755 on Unices.

Test code mostly written by Trott:
https://github.com/nodejs/node/issues/3392#issuecomment-148634126.

PR-URL: https://github.com/nodejs/node/pull/3394
Fixes: https://github.com/nodejs/node/issues/3392


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>