v.io/jiri/runutil: allow invocation of tool with ttys for stdout/stderr
v.io/jiri/runutil: allow invocation of tool with ttys for stdout/stderr

This CL allows jiri to invoke tools using the original os.File for
stdout and stderr, and thus allow an editor to get jiri's original tty
as stdout or stdin.

Change-Id: Iae44aacfa2c809c43dc32a10a5ce4409ec6ba6f0

vanadium/go.jiri,vanadium/go.jiri