VMS: modernise rand_pool_acquire_entropy, step 1

Stop redefining structures that are already defined in system
headers.  This also means we can stop setting the pointer size
globally, because the system structures will have the correct pointer
sizes either way.  The only exception is passing the right pointer
size to a function.

Stop trying to twist things around with rand(), that's the job of the
DRBG that we feed.

Stop assuming the location of the JPI$_FINALEXC item, look it up
instead.

Signal an exception if the sys$getjpiw call fails (it means the item
list isn't set up right, so works as an assertion, but using VMS
methodology).

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6151)