httpd: send Connection: close (single-connection uIP mitigation)

Real root cause of the "web login fails from a browser but works from curl":
the uIP httpd is built with UIP_CONF_MAX_CONNECTIONS = 1 and uses global
response state (outbuf/slen/session), i.e. it serves exactly one TCP connection
at a time and closes it after each response - but never advertises that via
the Connection header. A browser's HTTP/1.1 client therefore assumes the
connection may be persistent and can park it in its keep-alive pool for reuse;
a later request sent on that pooled connection hits one the server has already
closed, and a POST (unlike a GET) is never retried by the browser, so it can
be silently lost this way.

This adds "Connection: close" to every response so the browser does not pool
and reuse a connection the server is about to drop. On its own this did not
fully explain the reported login failures - the actual authentication bug is
fixed in the next commit (the Cookie header parsed at a fixed offset) - but it
is correct behaviour for a server that only ever handles one connection, and
removes one source of dropped requests.

httpd/page_impl.c

@@ -45,7 +45,7 @@ extern __xdata char sfp_module_model[2][17];
 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\nContent-Type: application/json\r\n\r\n";
+__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";
 
 // Convert uint8_t to ascii HEX char push on html-buffer.