Prevent the commands output of pkg_version.pl from being executed without the
user actually editing the output. Too many people were rampantly abusing
this feature via "pkg_version -c | sh" without really being cognizant
of the dangers involved (ports upgrade kits) or the fact that it
just plain wasn't designed for it (dependencies). We'll try to keep
people from shooting themselves in the foot.

Will be MFC-ed to RELENG_4 and RELENG_3 after cooling-off period.