Lowercase configuration arguments on VMS

Depending on user preferences, Configure might get something like
--PREFIX=blah just as well as --prefix=blah, or "SHARED" just as well
as "shared".  On VMS, let's therefore lowercase at least the portion
of the argument before a possible equal sign.

For good measure, we lowercase the arguments to be checked in
config.com as well.  The original argument is sent on to Configure,
however.

Reviewed-by: Andy Polyakov <appro@openssl.org>