VMS: rather use a quick file comparison than DIFF

VMS: rather use a quick file comparison than DIFF

VMS DIFF tries to calculate all the differences, which is slower than
just reading the files and stopping at the first difference.  The
latter doesn't exist as a command, so the problem is solved with perl
and File::Compare (has been in core perl since very early version 5).

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>