Make key exchange strength available.

This change stores the size of the group/modulus (for RSA/DHE) or curve
ID (for ECDHE) in the |SSL_SESSION|. This makes it available for UIs
where desired.

Change-Id: I354141da432a08f71704c9683f298b361362483d
Reviewed-on: https://boringssl-review.googlesource.com/5280
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/internal.h b/ssl/internal.h
index 7d4214d..63046e8 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -1072,6 +1072,11 @@
 int tls1_ec_curve_id2nid(uint16_t curve_id);
 int tls1_ec_nid2curve_id(uint16_t *out_curve_id, int nid);
 
+/* tls1_ec_curve_id2name returns a human-readable name for the
+ * curve specified by the TLS curve id in |curve_id|. If the
+ * curve is unknown, it returns NULL. */
+const char* tls1_ec_curve_id2name(uint16_t curve_id);
+
 /* tls1_check_curve parses ECParameters out of |cbs|, modifying it. It
  * checks the curve is one of our preferences and writes the
  * NamedCurve value to |*out_curve_id|. It returns one on success and