DSO: Fix the VMS DSO name converter to actually do something

DSO: Fix the VMS DSO name converter to actually do something

This function has never before actually done its work.  This wasn't
discovered before, because its output wasn't important before the FIPS
provider self test started using its value.

This function is now made to insert the VMS DSO extension (".EXE") at
the end of the filename, being careful to make sure what can be a
typical VMS generation number (separated from the file name with a
';') remains at the end.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15765)