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_stp.c

@@ -15,6 +15,8 @@
 #include "uip.h"
 #include "machine.h"
 
+extern __xdata uint8_t err_status;
+
 // All entry points are __banked and nothing here runs from an interrupt,
 // so the module does not need to stay in the resident bank
 #pragma codeseg BANK2
@@ -1134,5 +1136,6 @@ void stp_parse(void) __banked __reentrant
 	}
 	return;
 err:
+	err_status = ERR_INVALID_ARGUMENT;
 	print_string("Error: stp on|off|status | prio <0-15> | hello <1-10> | maxage <6-40> | fwd <4-30> | txhold <1-10> | version rstp|stp | port <1-9>|lag <1-4> on|off|edge|cost|prio|guard|filter ...\n");
 }