MINOR: activity: rename confusing poll_* fields in the output

We have poll_drop, poll_dead and poll_skip which are confusingly named
like their poll_io and poll_exp counterparts except that they are not
per poll() call but per-fd. This patch renames them to poll_drop_fd(),
poll_dead_fd() and poll_skip_fd() for this reason.
diff --git a/src/cli.c b/src/cli.c
index 1939ec3..528f935 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1131,9 +1131,9 @@
 	chunk_appendf(&trash, "wake_signal:");  SHOW_TOT(thr, activity[thr].wake_signal);
 	chunk_appendf(&trash, "poll_io:");      SHOW_TOT(thr, activity[thr].poll_io);
 	chunk_appendf(&trash, "poll_exp:");     SHOW_TOT(thr, activity[thr].poll_exp);
-	chunk_appendf(&trash, "poll_drop:");    SHOW_TOT(thr, activity[thr].poll_drop);
-	chunk_appendf(&trash, "poll_dead:");    SHOW_TOT(thr, activity[thr].poll_dead);
-	chunk_appendf(&trash, "poll_skip:");    SHOW_TOT(thr, activity[thr].poll_skip);
+	chunk_appendf(&trash, "poll_drop_fd:"); SHOW_TOT(thr, activity[thr].poll_drop_fd);
+	chunk_appendf(&trash, "poll_dead_fd:"); SHOW_TOT(thr, activity[thr].poll_dead_fd);
+	chunk_appendf(&trash, "poll_skip_fd:"); SHOW_TOT(thr, activity[thr].poll_skip_fd);
 	chunk_appendf(&trash, "conn_dead:");    SHOW_TOT(thr, activity[thr].conn_dead);
 	chunk_appendf(&trash, "stream:");       SHOW_TOT(thr, activity[thr].stream);
 	chunk_appendf(&trash, "pool_fail:");    SHOW_TOT(thr, activity[thr].pool_fail);