Don't use EV_CLEAR for kqueue events.

For the semantics to match the epoll implementation, we need a socket to
continue to appear readable/writable if you wait multiple times without
doing I/O in between (in Linux terminology: level-triggered rather than
edge-triggered).  This distinction will be important for later commits.
Similar to commit 3b790d25 for Windows.
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJAC4Oqao%3DqforhNey20J8CiG2R%3DoBPqvfR0vOJrFysGw%40mail.gmail.com

src/backend/storage/ipc/latch.c

@@ -991,7 +991,7 @@ WaitEventAdjustKqueueAdd(struct kevent *k_ev, int filter, int action,
 {
 	k_ev->ident = event->fd;
 	k_ev->filter = filter;
-	k_ev->flags = action | EV_CLEAR;
+	k_ev->flags = action;
 	k_ev->fflags = 0;
 	k_ev->data = 0;
 	AccessWaitEvent(k_ev) = event;
@@ -1003,7 +1003,7 @@ WaitEventAdjustKqueueAddPostmaster(struct kevent *k_ev, WaitEvent *event)
 	/* For now postmaster death can only be added, not removed. */
 	k_ev->ident = PostmasterPid;
 	k_ev->filter = EVFILT_PROC;
-	k_ev->flags = EV_ADD | EV_CLEAR;
+	k_ev->flags = EV_ADD;
 	k_ev->fflags = NOTE_EXIT;
 	k_ev->data = 0;
 	AccessWaitEvent(k_ev) = event;