sandbox: Don't disable ctrlc() on sandbox if in raw mode

In raw mode, handle ctrl-c as normal. This allows normal ctrl-c behavior
such as aborting a command that is timing out without completely
terminating the sandbox executable.

In [1], Simon disabled this.  His reason for it was that it interferes
with piping test scripts. Piping should be done in cooked mode, so this
change should still not interfere.

[1] commit 8969ea3e ("sandbox: Disable Ctrl-C")

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>