sd-event: don't touch fd's accross forks

We protect most of the API from use accross forks, but we still allow both
sd_event and sd_event_source objects to be unref'ed. This would cause
problems as it would unregister sources from the underlying eventfd, hence
also affecting the original instance in the parent process.

This fixes the issue by not touching the fds on unref when done accross a fork,
but still free the memory.

This fixes a regression introduced by
        "udevd: move main-loop to sd-event": 693d371d30

where the worker processes were disabling the inotify event source in the
main daemon.

src/libsystemd/sd-event/sd-event.c

@@ -474,6 +474,9 @@ static int source_io_unregister(sd_event_source *s) {
         assert(s);
         assert(s->type == SOURCE_IO);
 
+        if (event_pid_changed(s->event))
+                return 0;
+
         if (!s->io.registered)
                 return 0;
 
@@ -604,6 +607,9 @@ static int event_update_signal_fd(sd_event *e) {
 
         assert(e);
 
+        if (event_pid_changed(e))
+                return 0;
+
         add_to_epoll = e->signal_fd < 0;
 
         r = signalfd(e->signal_fd, &e->sigset, SFD_NONBLOCK|SFD_CLOEXEC);