Preserve filename case on VMS.

Preserve filename case on VMS.

VMS systems of the last decade or so have the ability, when
operating on an ODS-5 volume, to preserve filename case.  In order
for CRTL calls to make use of this capability, the feature setting
DECC$EFS_CASE_PRESERVE must be explicitly enabled.  We now do that
for Perl at start-up time, which depends on latent support that
has been in the core since 5.10 but not quite working until
recently.

Note that case preservation of command-line arguments is not
included in this feature.  That's a different setting and is not
entirely under Perl's control as the command line is parsed by
DCL before the CRTL sees it.