etcp: per-link acked bytes/packets counters + BBR debug logging

- Add acked_bytes/acked_packets to struct ETCP_LINK (init, reset, increment per ACK)
- Add DEBUG_CATEGORY_BBR (23) for BBR-specific debug output
- Add BBR input/output DEBUG_DEBUG around bbr_main call in etcp_ack_recv
- Add "bbr" category name to debug_config and etcpmon GUI

lib/debug_config.c

@@ -127,6 +127,7 @@ static const struct {
     {"nat", DEBUG_CATEGORY_NAT},
     {"keepalive", DEBUG_CATEGORY_KEEPALIVE},
     {"etcp_route", DEBUG_CATEGORY_ETCPROUTE},
+    {"bbr", DEBUG_CATEGORY_BBR},
     {"all", DEBUG_CATEGORY_ALL},
     {NULL, DEBUG_CATEGORY_NONE}
 };