Since util-linux 2.31, runuser has has support (sometimes
undocumented) for defense against terminal hijacking via TIOCSTI or
other techniques, by passing a --pty argument.

That option is [no longer
experimental](https://github.com/karelzak/util-linux/issues/760

), so
monkeysphere-authentication should use it if it is available.

This defends against a compromised monkeysphere user being able to
attack the superuser when invoked under a controlling terminal.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>