Move in_handshake into STATEM

The SSL variable |in_handshake| seems misplaced. It would be better to have
it in the STATEM structure.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/d1_lib.c

@@ -1017,7 +1017,7 @@ int dtls1_heartbeat(SSL *s)
     }
 
     /* ...and no handshake in progress. */
-    if (SSL_in_init(s) || s->in_handshake) {
+    if (SSL_in_init(s) || ossl_statem_get_in_handshake(s)) {
         SSLerr(SSL_F_DTLS1_HEARTBEAT, SSL_R_UNEXPECTED_MESSAGE);
         return -1;
     }