Function mount 

pub fn mount<P1, P2, P3, P4>(
    source: Option<&P1>,
    target: &P2,
    fstype: Option<&P3>,
    flags: MsFlags,
    data: Option<&P4>,
) -> Result<(), Errno>
where
    P1: NixPath + ?Sized,
    P2: NixPath + ?Sized,
    P3: NixPath + ?Sized,
    P4: NixPath + ?Sized,