The interface has one method returning the `FileInfo` and a flag telling if `Lstat` was called or not.
```go
type Lstater interface {
LstatIfPossible(name string) (os.FileInfo, bool, error)
}
```
`Lstat` is currently only supported by the `OsFs`, but since that `Fs` can be used in others, they will also support it by proxy.
But not always, so hence this optional interface.
The interface is in this commit implemented for:
* BasePathFs
* OsFs
* CopyOnWriteFs
* ReadOnlyFs
Fixes#75
Bjørn Erik Pedersen