Preserve case of command-line arguments on VMS.

Preserve case of command-line arguments on VMS.

OpenVMS releases of the last decade or so have had this capability
but it's still not the default.  But it's a reasonable default for
Perl, so enable it in our initialization code.

Unfortunately this feature does not work unless extended parse has
been enabled in the process before invoking Perl.  Enabling
extended parse in our initialization code doesn't do any good
because DCL has already parsed the arguments before we get there.

So we will be limited to documenting that things work better with
extended parse and that the test suite assumes it's enabled.