Modify 'bssl client' to print the cert subject and issuer

This is the one piece of functionality I miss from the openssl tool -
the ability to see some basic information about the server cert.

Sample output:
==========
$ bssl client -connect www.google.com
Connecting to [2607:f8b0:4006:80d::1010]:443
Connected.
  Version: TLSv1.2
  Resumed session: no
  Cipher: ECDHE-RSA-AES128-GCM-SHA256
  ECDHE curve: P-256
  Secure renegotiation: yes
  Next protocol negotiated:
  ALPN protocol:
  Cert subject: /C=US/ST=California/L=Mountain View/O=Google Inc/CN=www.google.com
  Cert issuer: /C=US/O=Google Inc/CN=Google Internet Authority G2
==========

Change-Id: I758682784752a616628138e420f52586d5a1bb31
Reviewed-on: https://boringssl-review.googlesource.com/7620
Reviewed-by: David Benjamin <davidben@google.com>

tool/transport_common.cc

@@ -44,6 +44,7 @@ typedef int ssize_t;
 
 #include <openssl/err.h>
 #include <openssl/ssl.h>
+#include <openssl/x509.h>
 
 #include "internal.h"
 #include "transport_common.h"
@@ -191,6 +192,19 @@ void PrintConnectionInfo(const SSL *ssl) {
   unsigned alpn_len;
   SSL_get0_alpn_selected(ssl, &alpn, &alpn_len);
   fprintf(stderr, "  ALPN protocol: %.*s\n", alpn_len, alpn);
+
+  // Print the server cert subject and issuer names.
+  X509 *peer = SSL_get_peer_certificate(ssl);
+  if (peer != NULL) {
+    fprintf(stderr, "  Cert subject: ");
+    X509_NAME_print_ex_fp(stderr, X509_get_subject_name(peer), 0,
+                          XN_FLAG_ONELINE);
+    fprintf(stderr, "\n  Cert issuer: ");
+    X509_NAME_print_ex_fp(stderr, X509_get_issuer_name(peer), 0,
+                          XN_FLAG_ONELINE);
+    fprintf(stderr, "\n");
+    X509_free(peer);
+  }
 }
 
 bool SocketSetNonBlocking(int sock, bool is_non_blocking) {