VMS: be less picky when loading DSOs

The DSO API was picky about casing of symbol names on VMS.

There's really no reason to be that picky, it's mostly just annoying.
Therefore, we take away the possibility to flag for a choice, and will
instead first try to find a symbol with exact case, and failing that,
we try to find it in upper case.

Reviewed-by: Rich Salz <rsalz@openssl.org>