Add SSL_SESSION_is_single_use.

Querying versions is a bit of a mess between DTLS and TLS and variants
and friends. Add SSL_SESSION_is_single_use which informs the caller
whether the session should be single-use.

Bug: chromium:631988
Change-Id: I745d8a5dd5dc52008fe99930d81fed7651b92e4e
Reviewed-on: https://boringssl-review.googlesource.com/20844
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
diff --git a/ssl/ssl_session.cc b/ssl/ssl_session.cc
index 4c6d93f..5da24c4 100644
--- a/ssl/ssl_session.cc
+++ b/ssl/ssl_session.cc
@@ -960,6 +960,10 @@
   return 1;
 }
 
+int SSL_SESSION_should_be_single_use(const SSL_SESSION *session) {
+  return SSL_SESSION_protocol_version(session) >= TLS1_3_VERSION;
+}
+
 SSL_SESSION *SSL_magic_pending_session_ptr(void) {
   return (SSL_SESSION *)&g_pending_session_magic;
 }