httpd: clear upload reset intent when the TCP slot is reused

With UIP_CONF_MAX_CONNECTIONS set to one, a verified upload can leave
reset intent set when its response is never acknowledged. A subsequent
TCP connection reuses that slot, and ACKing its plain GET response can
otherwise trigger reset_chip(). This is sequential slot reuse, not
concurrent HTTP connections.

Store reset intent in the connection appstate and initialize both tstate
and fw_reset_pending on every uip_connected() event, regardless of stale
appstate contents. Reset only after the verified upload response is fully
acknowledged.

Exercise the real HTTP callback for all stale tstate bytes, sequential
slot reuse and a later GET, partial/final ACKs, abort, timeout and CRC
failure. Keep the regression tests in the existing tab-indented style.

httpd/httpd.h

@@ -12,6 +12,7 @@
    for each TCP connection. */
 typedef struct httpd_state {
    uint8_t tstate;
+   uint8_t fw_reset_pending;
 } uip_tcp_appstate_t;
 
 /* Finally we define the application function to be called by uIP. */