Rename X.509 members in |SSL_SESSION| and |CERT|.

This change renames |peer| to |x509_peer| and |cert_chain| to
|x509_chain| in |SSL_SESSION|. It also renames |x509| to |x509_leaf| and
|chain| to |x509_chain| in |CERT|. (All with an eye to maybe making
them lazily initialised in the future).

This a) catches anyone who might be accessing these members directly and
b) makes space for |CRYPTO_BUFFER|-based values to take the unprefixed
names.

Change-Id: I10573304fb7d6f1ea03f9e645f7fc0acdaf71ac2
Reviewed-on: https://boringssl-review.googlesource.com/12162
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/ssl/internal.h b/ssl/internal.h
index 7badc5b..6b57971 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -1171,10 +1171,9 @@
 };
 
 typedef struct cert_st {
-  X509 *x509;
   EVP_PKEY *privatekey;
-  /* Chain for this certificate */
-  STACK_OF(X509) *chain;
+  X509 *x509_leaf;
+  STACK_OF(X509) *x509_chain;
 
   /* key_method, if non-NULL, is a set of callbacks to call for private key
    * operations. */