wintun: do not load dll in init()

This prevents linking to wintun.dll until it's actually needed, which
should improve startup time.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/wintun/dll_fromfile_windows.go

@@ -20,6 +20,7 @@ type lazyDLL struct {
 	Name   string
 	mu     sync.Mutex
 	module windows.Handle
+	onLoad func(d *lazyDLL)
 }
 
 func (d *lazyDLL) Load() error {
@@ -42,6 +43,9 @@ func (d *lazyDLL) Load() error {
 	}
 
 	atomic.StorePointer((*unsafe.Pointer)(unsafe.Pointer(&d.module)), unsafe.Pointer(module))
+	if d.onLoad != nil {
+		d.onLoad(d)
+	}
 	return nil
 }