Add per-SSL 'msg_callback' with 'msg_callback_arg'.

Both have per-SSL_CTX defaults.
These new values can be set by calling SSL[_CTX]_[callback_]ctrl
with codes SSL_CTRL_SET_MSG_CALLBACK and SSL_CTRL_SET_MSG_CALLBACK_ARG.

So far, the callback is never actually called.


Also rearrange some SSL_CTX struct members (some exist just in
SSL_CTXs, others are defaults for SSLs and are either copied
during SSL_new, or used if the value in the SSL is not set;
these three classes of members were not in a logical order),
and add some missing assignments to SSL_dup.