wintun: remove memmod option for dll loading

Only wireguard-windows used this, and it's moving to wgnt exclusively.

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

tun/wintun/dll_windows.go

@@ -10,6 +10,8 @@ import (
 	"sync"
 	"sync/atomic"
 	"unsafe"
+
+	"golang.org/x/sys/windows"
 )
 
 func newLazyDLL(name string, onLoad func(d *lazyDLL)) *lazyDLL {
@@ -57,3 +59,40 @@ func (p *lazyProc) Addr() uintptr {
 	}
 	return p.addr
 }
+
+type lazyDLL struct {
+	Name   string
+	mu     sync.Mutex
+	module windows.Handle
+	onLoad func(d *lazyDLL)
+}
+
+func (d *lazyDLL) Load() error {
+	if atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&d.module))) != nil {
+		return nil
+	}
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	if d.module != 0 {
+		return nil
+	}
+
+	const (
+		LOAD_LIBRARY_SEARCH_APPLICATION_DIR = 0x00000200
+		LOAD_LIBRARY_SEARCH_SYSTEM32        = 0x00000800
+	)
+	module, err := windows.LoadLibraryEx(d.Name, 0, LOAD_LIBRARY_SEARCH_APPLICATION_DIR|LOAD_LIBRARY_SEARCH_SYSTEM32)
+	if err != nil {
+		return fmt.Errorf("Unable to load library: %w", err)
+	}
+
+	atomic.StorePointer((*unsafe.Pointer)(unsafe.Pointer(&d.module)), unsafe.Pointer(module))
+	if d.onLoad != nil {
+		d.onLoad(d)
+	}
+	return nil
+}
+
+func (p *lazyProc) nameToAddr() (uintptr, error) {
+	return windows.GetProcAddress(p.dll.module, p.Name)
+}