TLS: reject duplicate extensions

Adapted from BoringSSL. Added a test.

The extension parsing code is already attempting to already handle this for
some individual extensions, but it is doing so inconsistently. Duplicate
efforts in individual extension parsing will be cleaned up in a follow-up.

Reviewed-by: Stephen Henson <steve@openssl.org>

ssl/ssl_err.c

diff --git a/ssl/ssl_err.c b/ssl/ssl_err.c
index e6b9bbd..46f483f 100644
--- a/ssl/ssl_err.c
+++ b/ssl/ssl_err.c
@@ -273,6 +273,8 @@
     {ERR_FUNC(SSL_F_STATE_MACHINE), "state_machine"},
     {ERR_FUNC(SSL_F_TLS12_CHECK_PEER_SIGALG), "tls12_check_peer_sigalg"},
     {ERR_FUNC(SSL_F_TLS1_CHANGE_CIPHER_STATE), "tls1_change_cipher_state"},
+    {ERR_FUNC(SSL_F_TLS1_CHECK_DUPLICATE_EXTENSIONS),
+     "tls1_check_duplicate_extensions"},
     {ERR_FUNC(SSL_F_TLS1_CHECK_SERVERHELLO_TLSEXT),
      "TLS1_CHECK_SERVERHELLO_TLSEXT"},
     {ERR_FUNC(SSL_F_TLS1_EXPORT_KEYING_MATERIAL),