Add method to query Extended Master Secret support

Change-Id: I4c285f4b3dd77f8fc7249c7504b5e4eb9b62959f
Reviewed-on: https://boringssl-review.googlesource.com/5920
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/ssl.h

diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index 63a0bdd..88f11f9 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -962,6 +962,10 @@
 OPENSSL_EXPORT int SSL_get_tls_unique(const SSL *ssl, uint8_t *out,
                                       size_t *out_len, size_t max_out);
 
+/* SSL_get_extms_support returns one if the Extended Master Secret
+ * extension was negotiated. Otherwise, it returns zero. */
+OPENSSL_EXPORT int SSL_get_extms_support(const SSL *ssl);
+
 
 /* Custom extensions.
  *