TUN inbound: Add FreeBSD support (#5891)

And reverts "refactor `mtu` to support setting IPv4/v6 separately" https://github.com/XTLS/Xray-core/pull/5891#issuecomment-4245677624

And fixes `autoOutboundsInterface` on Windows https://github.com/XTLS/Xray-core/pull/5887#issuecomment-4251719900

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Co-authored-by: LjhAUMEM <llnu14702@gmail.com>

proxy/tun/tun_android.go

@@ -73,7 +73,7 @@ func (t *AndroidTun) Index() (int, error) {
 func (t *AndroidTun) newEndpoint() (stack.LinkEndpoint, error) {
 	return fdbased.New(&fdbased.Options{
 		FDs:               []int{t.tunFd},
-		MTU:               t.options.MTU[0],
+		MTU:               t.options.MTU,
 		RXChecksumOffload: true,
 	})
 }