Add ssl_renegotiate_ignore.

This option causes clients to ignore HelloRequest messages completely.
This can be suitable in cases where a server tries to perform concurrent
application data and handshake flow, e.g. because they are trying to
“renew” symmetric keys.

Change-Id: I2779f7eff30d82163f2c34a625ec91dc34fab548
Reviewed-on: https://boringssl-review.googlesource.com/6431
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

ssl/s3_pkt.c

diff --git a/ssl/s3_pkt.c b/ssl/s3_pkt.c
index c50b315..7416d0e 100644
--- a/ssl/s3_pkt.c
+++ b/ssl/s3_pkt.c
@@ -346,6 +346,8 @@
       return ssl->s3->total_renegotiations == 0;
     case ssl_renegotiate_freely:
       return 1;
+    case ssl_renegotiate_ignore:
+      return 1;
   }
 
   assert(0);
@@ -567,6 +569,10 @@
       goto err;
     }
 
+    if (s->renegotiate_mode == ssl_renegotiate_ignore) {
+      goto start;
+    }
+
     /* Renegotiation is only supported at quiescent points in the application
      * protocol, namely in HTTPS, just before reading the HTTP response. Require
      * the record-layer be idle and avoid complexities of sending a handshake