dtls_get_message changes for state machine move

Create a dtls_get_message function similar to the old dtls1_get_message but
in the format required for the new state machine code. The old function will
eventually be deleted in later commits.

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

include/openssl/ssl.h

@@ -1928,6 +1928,7 @@ void ERR_load_SSL_strings(void);
 # define SSL_F_DTLS1_SEND_SERVER_HELLO                    266
 # define SSL_F_DTLS1_SEND_SERVER_KEY_EXCHANGE             267
 # define SSL_F_DTLS1_WRITE_APP_DATA_BYTES                 268
+# define SSL_F_DTLS_GET_REASSEMBLED_MESSAGE               370
 # define SSL_F_READ_STATE_MACHINE                         352
 # define SSL_F_SSL3_ACCEPT                                128
 # define SSL_F_SSL3_ADD_CERT_TO_BUF                       296