MINOR: cli/activity: add 3 general purpose counters in development mode

The unused fd_del and fd_skip were being abused during debugging sessions
as general purpose event counters. With their removal, let's officially
have dedicated counters for such use cases. These counters are called
"ctr0".."ctr2" and are listed at the end when DEBUG_DEV is set.
diff --git a/src/cli.c b/src/cli.c
index 118f8fd..39e246e 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1096,6 +1096,12 @@
 	chunk_appendf(&trash, "\naccq_ring:");    for (thr = 0; thr < global.nbthread; thr++) chunk_appendf(&trash, " %u", (accept_queue_rings[thr].tail - accept_queue_rings[thr].head + ACCEPT_QUEUE_SIZE)%ACCEPT_QUEUE_SIZE);
 #endif
 
+#if defined(DEBUG_DEV)
+	/* keep these ones at the end */
+	chunk_appendf(&trash, "\nctr0:");         for (thr = 0; thr < global.nbthread; thr++) chunk_appendf(&trash, " %u", activity[thr].ctr0);
+	chunk_appendf(&trash, "\nctr1:");         for (thr = 0; thr < global.nbthread; thr++) chunk_appendf(&trash, " %u", activity[thr].ctr1);
+	chunk_appendf(&trash, "\nctr2:");         for (thr = 0; thr < global.nbthread; thr++) chunk_appendf(&trash, " %u", activity[thr].ctr2);
+#endif
 	chunk_appendf(&trash, "\n");
 
 	if (ci_putchk(si_ic(si), &trash) == -1) {