Provide functions to write early data

We provide SSL_write_early() which *must* be called first on a connection
(prior to any other IO function including SSL_connect()/SSL_do_handshake()).
Also SSL_write_early_finish() which signals the end of early data.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2737)

ssl/s3_msg.c

diff --git a/ssl/s3_msg.c b/ssl/s3_msg.c
index 743a02b..7af2f99 100644
--- a/ssl/s3_msg.c
+++ b/ssl/s3_msg.c
@@ -63,7 +63,10 @@
 int ssl3_send_alert(SSL *s, int level, int desc)
 {
     /* Map tls/ssl alert value to correct one */
-    desc = s->method->ssl3_enc->alert_value(desc);
+    if (SSL_TREAT_AS_TLS13(s))
+        desc = tls13_alert_code(desc);
+    else
+        desc = s->method->ssl3_enc->alert_value(desc);
     if (s->version == SSL3_VERSION && desc == SSL_AD_PROTOCOL_VERSION)
         desc = SSL_AD_HANDSHAKE_FAILURE; /* SSL 3.0 does not have
                                           * protocol_version alerts */