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.h

@@ -3,6 +3,8 @@
 
 #include <stdbool.h>
 
+extern __code uint8_t * __code HTTP_RESPONCE_TXT;
+
 bool send_counters(uint8_t phys_port);
 void send_status(void);
 void send_vlan(uint16_t vlan);