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/ev_epoll.c b/src/ev_epoll.c
index e5437a2..5102b10 100644
--- a/src/ev_epoll.c
+++ b/src/ev_epoll.c
@@ -142,7 +142,7 @@
 
 		_HA_ATOMIC_AND(&fdtab[fd].update_mask, ~tid_bit);
 		if (!fdtab[fd].owner) {
-			activity[tid].poll_drop++;
+			activity[tid].poll_drop_fd++;
 			continue;
 		}
 
@@ -208,13 +208,13 @@
 		fd = epoll_events[count].data.fd;
 
 		if (!fdtab[fd].owner) {
-			activity[tid].poll_dead++;
+			activity[tid].poll_dead_fd++;
 			continue;
 		}
 
 		if (!(fdtab[fd].thread_mask & tid_bit)) {
 			/* FD has been migrated */
-			activity[tid].poll_skip++;
+			activity[tid].poll_skip_fd++;
 			epoll_ctl(epoll_fd[tid], EPOLL_CTL_DEL, fd, &ev);
 			_HA_ATOMIC_AND(&polled_mask[fd].poll_recv, ~tid_bit);
 			_HA_ATOMIC_AND(&polled_mask[fd].poll_send, ~tid_bit);