unix: add example for Flock

The method signature does not explain much about how to use the
function, and which arguments to invoke it with, so an example would
be helpful.

Change-Id: I95747567b2cafe70da1e04939c39f6b507f12684
Reviewed-on: https://go-review.googlesource.com/130317Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/example_test.go

@@ -17,3 +17,14 @@ func ExampleExec() {
 	err := unix.Exec("/bin/ls", []string{"ls", "-al"}, os.Environ())
 	log.Fatal(err)
 }
+
+func ExampleFlock() {
+	f, _ := os.Create("example.lock")
+	if err := unix.Flock(int(f.Fd()), unix.LOCK_EX); err != nil {
+		log.Fatal(err)
+	}
+	// Do work here that requires the lock. When finished, release the lock:
+	if err := unix.Flock(int(f.Fd()), unix.LOCK_UN); err != nil {
+		log.Fatal(err)
+	}
+}