Implement Token Binding

Update-Note: Token Binding can no longer be configured with the custom
  extensions API. Instead, use the new built-in implementation. (The
  internal repository should be all set.)

Bug: 183

Change-Id: I007523a638dc99582ebd1d177c38619fa7e1ac38
Reviewed-on: https://boringssl-review.googlesource.com/20645
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/tls1.h

diff --git a/include/openssl/tls1.h b/include/openssl/tls1.h
index 4b5d226..682bb9b 100644
--- a/include/openssl/tls1.h
+++ b/include/openssl/tls1.h
@@ -202,6 +202,9 @@
 // ExtensionType value from RFC7627
 #define TLSEXT_TYPE_extended_master_secret 23
 
+// ExtensionType value from draft-ietf-tokbind-negotiation-10
+#define TLSEXT_TYPE_token_binding 24
+
 // ExtensionType value from RFC4507
 #define TLSEXT_TYPE_session_ticket 35