httpd: answer /cmd with the shared text header

The /cmd path carried its own copy of the plain-text response header,
which was the same string page_impl.c already keeps in
HTTP_RESPONCE_TXT, minus one difference: the shared one never said
Connection: close.

This httpd closes after every response, so a reply that does not
advertise it invites the browser to reuse the socket and lose the next
request. HTTP_RESPONCE_JSON has always said so; the text one was the
odd one out, and send_config() and send_cmd_log() were answering
without it.

The copy is gone and the shared string carries the header. The check
for a command that printed nothing now compares against the length
strtox() returned instead of sizeof() on the macro.

httpd/page_impl.c

@@ -48,7 +48,7 @@ extern __xdata char sfp_module_serial[2][17];
 extern __xdata uint8_t sfp_options[2];
 
 __code uint8_t * __code HTTP_RESPONCE_JSON = "HTTP/1.1 200 OK\r\nConnection: close\r\nContent-Type: application/json\r\n\r\n";
-__code uint8_t * __code HTTP_RESPONCE_TXT = "HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\n\r\n";
+__code uint8_t * __code HTTP_RESPONCE_TXT = "HTTP/1.1 200 OK\r\nConnection: close\r\nContent-Type: text/plain\r\n\r\n";
 
 // Convert uint8_t to ascii HEX char push on html-buffer.
 void charhex_to_html(char c)