net/tstun, tsnet: make happier on WASI

Also fix a js/wasm issue with tsnet in the process. (same issue as WASI)

Updates #8320
Fixes #8315

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tun.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !js
+//go:build !wasm
 
 // Package tun creates a tuntap device, working around OS-specific
 // quirks if necessary.