Reverse order of environ tables under bash on VMS.

The default for populating %ENV on VMS is from logical names
first and the CRTL environ array second unless overridden by
setting PERL_ENV_TABLES. That's a reasonable default for DCL,
but not quite right when running under bash as exports under bash
might end up hidden behind logical names.  So reverse the default
order when GNV$UNIX_SHELL is set.

Also stop mentioning that setenv() may not be present in the CRTL
since it is present for VMS versions 7.0 and later and those are
the only ones we support.