These callbacks aren't actually defined in ssl.h, but they were defined in that opentls PR.  I'm not entirely certain whether that's because they are needed by pyopenssl, or just to make things look nice.  I was guessing it's because it's needed by pyopenssl.

The function definition the opentls branch looks like this: `void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, verify_callback cb);`, but in ssl.h it looks like: `void SSL_CTX_set_verify(SSL_CTX *ctx,int mode, int (*callback)(int, X509_STORE_CTX *));`.

`verify_callback` isn't defined in ssl.h, and for some reason when I leave the definition just in TYPES, when I attempt to import the API module, I get errors about conflicting types in `SSL_CTX_set_verify`.  If I put it in the INCLUDES section to, so that it gets passed to the `verify` function in CFFI, those errors seem to go away?

I wasn't sure what to do here.  I could just eliminate the `verify_callback` definition and make the `SSL_CTX_set_verify` definition look the same as it does in ssl.h, but I thought maybe pyopenssl might use it?