The SSL session cache module (ssl_cache) now also retains peer_cert information (not the entire chain)

The real peer certificate is copied into a x509_buf in the
ssl_cache_entry and reinstated upon cache retrieval. The information
about the rest of the certificate chain is lost in the process.

As the handshake (and certificate verification) has already been
performed, no issue is foreseen.
diff --git a/include/polarssl/ssl_cache.h b/include/polarssl/ssl_cache.h
index 85e0ed1..10cff20 100644
--- a/include/polarssl/ssl_cache.h
+++ b/include/polarssl/ssl_cache.h
@@ -46,6 +46,7 @@
 {
     time_t timestamp;           /*!< entry timestamp    */
     ssl_session session;        /*!< entry session      */
+    x509_buf peer_cert;         /*!< entry peer_cert    */
     ssl_cache_entry *next;      /*!< chain pointer      */
 };