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/include/types/activity.h b/include/types/activity.h
index 2a6f5bd..618785d 100644
--- a/include/types/activity.h
+++ b/include/types/activity.h
@@ -54,6 +54,12 @@
 	unsigned int accepted;     // accepted incoming connections
 	unsigned int accq_pushed;  // accept queue connections pushed
 	unsigned int accq_full;    // accept queue connection not pushed because full
+#if defined(DEBUG_DEV)
+	/* keep these ones at the end */
+	unsigned int ctr0;         // general purposee debug counter
+	unsigned int ctr1;         // general purposee debug counter
+	unsigned int ctr2;         // general purposee debug counter
+#endif
 	char __pad[0]; // unused except to check remaining room
 	char __end[0] __attribute__((aligned(64))); // align size to 64.
 };
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) {