cmd: report a rejected command as an error and keep it out of the history

run_cmd_body() answers POST /cmd with 400 when err_status is set, but
only the tokenizer set it. Every "Error:", "Invalid ..." and "Unknown
command" from the parsers went back as 200, so the web page reported the
command as applied, and save_cmd put the rejected line into the command
history, from where Save to flash wrote it into the startup config.

Set ERR_INVALID_ARGUMENT on every path that rejects a command: the err:
labels of the parsers, the parsers that print their own error, the
stp parser and the unknown command fallback. Warnings that still apply
the command, such as a full VLAN name table, are left alone.

Record a command in the history only when it was accepted, and let
execute_commands() report the first failure of a multi-line body, since
cmd_tokenize() clears err_status again for every following line.

rtl837x_common.h

@@ -54,6 +54,7 @@ extern __xdata uint8_t sbuf[SBUF_SIZE];
 #define ERR_OK			0
 #define ERR_TOO_MANY_ARGUMENTS	1
 #define ERR_CMD_TOO_LONG	2
+#define ERR_INVALID_ARGUMENT	3
 
 // For RX data, a propriatary RTL FRAME is inserted. Instead of 0x0800 for IPv4,
 // the RTL_FRAME_TAG_ID is used as part of an 8-byte tag. When VLAN is activated,