Non-blocking functions currently can return special return values, ERR_SSL_WANT_READ (-1) and ERR_SSL_WANT_WRITE (-2). However, it turns out that also a third value, ERR_SSL_WOULD_BLOCK is needed.

New definitions should be:

ERR_SSL_WOULD_BLOCK = -1,
ERR_SSL_WANT_READ  = -2,
ERR_SSL_WANT_WRITE = -3

Currently in case of EWOULDBLOCK, functions would return ERR_SSL_WANT_WRITE, which is wrong.

Unfortunately, this breaks the API in a minor version, but given that non-blocking TLS functions weren't working properly before 1.4.0 anyway, this is acceptable.