Add mount2 syscall

[morr0ne]

Adds a mount2 syscall.

This is actually the same as the current mount syscall. The main difference being the function parameters which are arguably wrong in the original implementation since it did not account for certain values being null.

This pr is incomplete since the parameters have been changed from Arg to &CStr. This is not inline with other apis and should be fixed but I could not figure out how to convert the types properly. I temporarily used a different type because I needed to use the function. Help fixing the parameters would be appreciated.

[sunfishcode]

To fix the Arg types, would it work to add something like this to src/path/arg.rs and then use that in place of .into_with_c_str?

pub fn option_into_with_c_str<T, F, A: Arg>(a: Option<A>, f: F) -> io::Result<T>
where
    A: Sized,
    F: FnOnce(Option<&CStr>) -> io::Result<T>,
{
    if let Some(a) = a {
        a.into_with_c_str(|p| f(Some(p)))
    } else {
        f(None)
    }
}

[morr0ne]

It should work I believe. I was thinking of something similar but I wanted to wait for other opinions first

[morr0ne]

While testing I realized that that data should not be an Arg, the data is interpreted differently based on the filesystem being mounted so it can't be generalized to anything "Path-like". I changed it to a CStr witch is more inline with the syscall api