Driving & safemode fixes

In response to http://smf.cataclysmdda.com/index.php?topic=4541.0 .  Looks like vehicle-safemode may have been the culprit, but Web Weaver certainly contributes.  In any event, now safemode stops you from pausing when it's on and there's a hostile in sight.

I haven't been able to get it to give the message when driving a vehicle (regardless of the option's status), but at least now people are safe.

(I thought the engine-upgrade merited a Help update.  In light of #4770 , which is a better solution...maybe not.)