Added mbedtls_net_close and use it in ssl_fork_server to correctly

disassociate the client socket from the parent process and the server
socket from the child process.

include/mbedtls/net_sockets.h

@@ -257,6 +257,13 @@ int mbedtls_net_send( void *ctx, const unsigned char *buf, size_t len );
 int mbedtls_net_recv_timeout( void *ctx, unsigned char *buf, size_t len,
                       uint32_t timeout );
 
+/**
+ * \brief          Closes down the connection and free associated data
+ *
+ * \param ctx      The context to close
+ */
+void mbedtls_net_close( mbedtls_net_context *ctx );
+
 /**
  * \brief          Gracefully shutdown the connection and free associated data
  *