MINOR: activity: add per-thread statistics on FD takeover

The FD takeover operation might have certain impacts explaining
unexpected activities, so it's important to report such a counter
there. We thus count the number of times a thread has stolen an
FD from another thread.
diff --git a/src/cli.c b/src/cli.c
index 3438fff..c1964df 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1174,6 +1174,7 @@
 	chunk_appendf(&trash, "accq_full:");    SHOW_TOT(thr, activity[thr].accq_full);
 #ifdef USE_THREAD
 	chunk_appendf(&trash, "accq_ring:");    SHOW_TOT(thr, (accept_queue_rings[thr].tail - accept_queue_rings[thr].head + ACCEPT_QUEUE_SIZE) % ACCEPT_QUEUE_SIZE);
+	chunk_appendf(&trash, "fd_takeover:");  SHOW_TOT(thr, activity[thr].fd_takeover);
 #endif
 
 #if defined(DEBUG_DEV)