Use no-consuming methods for MountOptions

[asomers]

The consuming methods make for a nice compact syntax - as long as the programmer knows exactly which methods will be needed. But if some mount options depend on runtime options then the consuming methods are awkward:

let mut opts = MountOptions::default();
if let Some(foo) = cli.bar {
    opts = opts.custom_options(foo)
}

Non-consuming options, while requiring 1 more line in the simplest case, are more flexible.

While I'm here, remove some extraneous clone()s.