Windows - `customFlags` should use `filepath.Clean()` instead of `path.Clean()`

[tintinweb]

go-ethereum/cmd/utils/customflags.go is handling OS paths and therefore it should use filepath.Clean() (works for windows path separator) instead of path.Clean() (nix path separator)

go-ethereum/cmd/utils/customflags.go

Line 199 in fdb7424

return path.Clean(os.ExpandEnv(p))

path.Clean()
https://github.com/golang/go/blob/master/src/path/path.go#L51-L62

filepath.Clean()
https://github.com/golang/go/blob/master/src/path/filepath/path.go#L66-L79

Expected behaviour

should cleanup for OS path separator

Actual behaviour

path.Clean() assumes the path separator is / while it might be \\

[karalabe]

Good catch. Though we'd need to update that sole test too to pass on Windows if we update the test. I assume it will break because it explicitly hard codes Linux paths and expansions.