Adjust LPdir_unix.c on VMS for OpenSSL expectations

When OPENSSL_DIR_read implemented by LPdir_unix.c gets a Unixy path,
it will return file names like you'd expect them on Unix.

However, if given a path with VMS syntax, such as "[.foo]", it returns
file names with generation numbers, such as "bar.txt;1", which makes
sense for VMS expectations, but can be surprising for OpenSSL.

Our solution is to simply shave off the generation number if
OPENSSL_DIR_read() expects there should be one, and make sure not to
return the same file name twice.  Note that VMS filesystems are case
insensitive, so the check for duplicate file names are done without
regard to character case.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5587)