ssl verify mode flags: complete and convert to bitflags!

Add the server-only SslVerifyMode entries. Use bitflags! so we can or
them together easily.

I've updated the docs to mention server mode behavior, unfortunately
bitflags! doesn't seem to like `///` inside it, so I've replaced the
`///` with `//`.
bn: use null_mut instead of deprecated mut_null
ssl: use null_mut instead of deprecated mut_null
x509: add the api needed to obtain entries by nid

Also includes a large list of nids (from openssl/object.h). Might be
transcoding errors on my part there.