file systems: Move pathbuf_from_stdout to fsext.rs

[prsabahrami]

Hi!
I think it would be good to move the pathbuf_from_stdout to fsext.rs file as mentioned in a commend above the function as well inside the touch.rs file and make it public. It is a fairly useful function!

// TODO: this may be a good candidate to put in fsext.rs
/// Returns a PathBuf to stdout.
///
/// On Windows, uses GetFinalPathNameByHandleW to attempt to get the path
/// from the stdout handle.
fn pathbuf_from_stdout() -> Result<PathBuf, TouchError> {