CLI: Fix for long-lived sessions during high loads

When there is a continuos stream of CLI commands, cli_get_command()
always returns 1 (there is a new command). Anyway, the socket receive
buffer was reset only when there was no command at all, leading to a
strange behavior: after a while, the CLI receive buffer came to its end,
then read() was called with zero size buffer, it returned 0 which was
interpreted as EOF.

The patch fixes that by resetting the buffer position after each command
and moving remaining data at the beginning of buffer.

Thanks to Maria Matejka for examining the bug and for the original bugfix.

nest/cli.h

@@ -29,7 +29,7 @@ typedef struct cli {
   node n;				/* Node in list of all log hooks */
   pool *pool;
   void *priv;				/* Private to sysdep layer */
-  byte *rx_buf, *rx_pos, *rx_aux;	/* sysdep */
+  byte *rx_buf, *rx_pos;		/* sysdep */
   struct cli_out *tx_buf, *tx_pos, *tx_write;
   event *event;
   void (*cont)(struct cli *c);