DTLS revision.

Revise DTLS code. There was a *lot* of code duplication in the
DTLS code that generates records. This makes it harder to maintain and
sometimes a TLS update is omitted by accident from the DTLS code.

Specifically almost all of the record generation functions have code like
this:

some_pointer = buffer + HANDSHAKE_HEADER_LENGTH;
... Record creation stuff ...
set_handshake_header(ssl, SSL_MT_SOMETHING, message_len);

...

write_handshake_message(ssl);

Where the "Record creation stuff" is identical between SSL/TLS and DTLS or
in some cases has very minor differences.

By adding a few fields to SSL3_ENC to include the header length, some flags
and function pointers for handshake header setting and handshake writing the
code can cope with both cases.

(cherry picked from commit 173e72e64c)

ssl/t1_lib.c

@@ -140,6 +140,48 @@ SSL3_ENC_METHOD TLSv1_enc_data={
 	TLS_MD_SERVER_FINISH_CONST,TLS_MD_SERVER_FINISH_CONST_SIZE,
 	tls1_alert_code,
 	tls1_export_keying_material,
+	0,
+	SSL3_HM_HEADER_LENGTH,
+	ssl3_set_handshake_header,
+	ssl3_handshake_write
+	};
+
+SSL3_ENC_METHOD TLSv1_1_enc_data={
+	tls1_enc,
+	tls1_mac,
+	tls1_setup_key_block,
+	tls1_generate_master_secret,
+	tls1_change_cipher_state,
+	tls1_final_finish_mac,
+	TLS1_FINISH_MAC_LENGTH,
+	tls1_cert_verify_mac,
+	TLS_MD_CLIENT_FINISH_CONST,TLS_MD_CLIENT_FINISH_CONST_SIZE,
+	TLS_MD_SERVER_FINISH_CONST,TLS_MD_SERVER_FINISH_CONST_SIZE,
+	tls1_alert_code,
+	tls1_export_keying_material,
+	SSL_ENC_FLAG_EXPLICIT_IV,
+	SSL3_HM_HEADER_LENGTH,
+	ssl3_set_handshake_header,
+	ssl3_handshake_write
+	};
+
+SSL3_ENC_METHOD TLSv1_2_enc_data={
+	tls1_enc,
+	tls1_mac,
+	tls1_setup_key_block,
+	tls1_generate_master_secret,
+	tls1_change_cipher_state,
+	tls1_final_finish_mac,
+	TLS1_FINISH_MAC_LENGTH,
+	tls1_cert_verify_mac,
+	TLS_MD_CLIENT_FINISH_CONST,TLS_MD_CLIENT_FINISH_CONST_SIZE,
+	TLS_MD_SERVER_FINISH_CONST,TLS_MD_SERVER_FINISH_CONST_SIZE,
+	tls1_alert_code,
+	tls1_export_keying_material,
+	SSL_ENC_FLAG_EXPLICIT_IV|SSL_ENC_FLAG_SIGALGS,
+	SSL3_HM_HEADER_LENGTH,
+	ssl3_set_handshake_header,
+	ssl3_handshake_write
 	};
 
 long tls1_default_timeout(void)