Do environ key case consistently on VMS.

Do environ key case consistently on VMS.

For those %ENV elements based on the CRTL environ array, we've
always preserved case when setting them but done look-ups only
after upcasing the key first, which makes lower- or mixed-case
entries go missing.

So make them consistently case-preserved and in the docs
distinguish this behavior from the case-blind behavior of keys
for %ENV entries based on logical namees and DCL symbols, which
remains unchanged.