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_default.go

@@ -1,4 +1,4 @@
-//go:build !linux && !windows && !android && !darwin
+//go:build !linux && !windows && !android && !darwin && !freebsd
 
 package tun