net: add support for FileConn, FilePacketConn, FileListener on Plan 9

A new test reveals that Plan 9 doesn't support File{Conn,Listener,PacketConn}.

```
--- FAIL: TestFileConn (0.00s)
    file_test.go:121: write tcp 127.0.0.1:58090: write /net/tcp/26/data: i/o on hungup channel
FAIL
```

Just FYI: https://storage.googleapis.com/go-build-log/b7d5a4b1/plan9-386_d2e48151.log
