Add X509 version constants.

Add X509 version constants.

The X509 version APIs return the numerical values of the version
numbers, which are one off from the names. This is a bit confusing.
Where they don't get it wrong (accidentally making an "X509v4"
certificate), callers tend to try commenting every call site to explain
the mismatch, including in OpenSSL itself.

Define constants for these values, so code can be self-documenting and
callers are nudged towards the right values.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14549)