Renamed error_strerror() to the less conflicting polarssl_strerror()

Ability to keep old function error_strerror() as well with
POLARSSL_ERROR_STRERROR_BC. Also works with
POLARSSL_ERROR_STRERROR_DUMMY.

include/polarssl/config.h

@@ -325,6 +325,17 @@
  */
 #define POLARSSL_KEY_EXCHANGE_ECDHE_RSA_ENABLED
 
+/**
+ * \def POLARSSL_ERROR_STRERROR_BC
+ *
+ * Make available the backward compatible error_strerror() next to the
+ * current polarssl_strerror().
+ *
+ * Disable if you run into name conflicts and want to really remove the
+ * error_strerror()
+ */
+#define POLARSSL_ERROR_STRERROR_BC
+
 /**
  * \def POLARSSL_ERROR_STRERROR_DUMMY
  *