Allow ssl_renegotiate() to be called in a loop

Previously broken if waiting for network I/O in the middle of a re-handshake
initiated by the client.

include/polarssl/ssl.h

@@ -200,7 +200,8 @@
 #define SSL_VERIFY_REQUIRED             2
 
 #define SSL_INITIAL_HANDSHAKE           0
-#define SSL_RENEGOTIATION               1
+#define SSL_RENEGOTIATION               1   /* In progress */
+#define SSL_RENEGOTIATION_DONE          2   /* Done */
 
 #define SSL_LEGACY_RENEGOTIATION        0
 #define SSL_SECURE_RENEGOTIATION        1