udevadm: flush output after each monitor event

If you're using `udevadm monitor` from a script, without a tty, then
libc defaults to being fully-buffered, and won't flush stdout after
newlines.  This is fine for tools that dump a bunch of data and then
exit immediately.  It's a problem for tools like `udevadm monitor` which
have long pauses: the buffered data can get stuck in the buffer for an
unbounded amount of time.

In the Cockpit project we've been working around this for some time with
`stdbuf` which is a `LD_PRELOAD` hack to change the libc buffering
behaviour, but we'd like to stop doing that.

Let's make sure we flush the buffer after each event.

src/udev/udevadm-monitor.c

@@ -55,6 +55,8 @@ static int device_monitor_handler(sd_device_monitor *monitor, sd_device *device,
                 printf("\n");
         }
 
+        fflush(stdout);
+
         return 0;
 }
 

test/units/TEST-07-PID1.start-limit.sh

@@ -35,6 +35,9 @@ for i in {1..5}; do
     echo "Start #$i"
 
     systemctl stop --no-block systemd-resolved.service
+    # Wait for systemd-resolved in ExecStart= being stopped.
+    # shellcheck disable=SC2016
+    timeout 10 bash -c 'until [[ "$(systemctl show --property=MainPID --value systemd-resolved.service)" == 0 ]]; do sleep 0.1; done'
     if ! resolvectl; then
         journalctl -o short-monotonic --no-hostname --no-pager -u systemd-resolved.service -n 15
         exit 1