Add SSL_get_pending_cipher()

The existing function SSL_get_current_cipher() queries the
current session for the ciphersuite in use, but there is no way
for application code to determine what ciphersuite has been
negotiated and will be used in the future, prior to ChangeCipherState
(or the TLS 1.3 equivalent) causing the new cipher to take effect and
become visible in the session information.  Expose this information
to appropriate application callbacks to use during the handshake.

The name SSL_get_pending_cipher() was chosen for compatibility with
BoringSSL's routine of that name.

Improve the note on macro implementations in SSL_get_current_cipher.pod
while here.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4070)