Extended master secret support.

This change implements support for the extended master secret. See
https://tools.ietf.org/html/draft-ietf-tls-session-hash-01
https://secure-resumption.com/

Change-Id: Ifc7327763149ab0894b4f1d48cdc35e0f1093b93
Reviewed-on: https://boringssl-review.googlesource.com/1930
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/bytestring.h

diff --git a/include/openssl/bytestring.h b/include/openssl/bytestring.h
index 9d2b4a9..d5e6cf9 100644
--- a/include/openssl/bytestring.h
+++ b/include/openssl/bytestring.h
@@ -193,6 +193,14 @@
                                                 unsigned tag,
                                                 uint64_t default_value);
 
+/* CBS_get_optional_asn1_bool gets an optional, explicitly-tagged BOOLEAN from
+ * |cbs|. If present, it sets |*out| to either zero or one, based on the
+ * boolean. Otherwise, it sets |*out| to |default_value|. It returns one on
+ * success, whether or not the element was present, and zero on decode
+ * failure. */
+OPENSSL_EXPORT int CBS_get_optional_asn1_bool(CBS *cbs, int *out, unsigned tag,
+                                              int default_value);
+
 
 /* CRYPTO ByteBuilder.
  *