tail: handle events after tailed and before watcher starts watching

[niyaznigmatullin]

When --follow=name read again all the files after watcher has been set up, if file changed just before the watcher started watching and after tail has happened.

This is done only when not using polling.

Tested with this sleep: sleep after all the files tailed and before watcher has been set up. For instance F-headers.sh didn't work without the fix if this line added, but starts working after the fix.

fn follow(files: &mut FileHandling, settings: &mut Settings) -> UResult<()> {
+    thread::sleep(Duration::from_millis(4000));
    let mut process = platform::ProcessChecker::new(settings.pid);

Probably fixes #3765 even without changing the testcase.

[niyaznigmatullin]

@jhscheer, I'm interested in your thoughts on that