Mounted: add path resolution

[kolyshkin]

The argument to mount.Mounted call needs to be an absolute path with symlinks resolved and cleaned (otherwise it won't match the path in mountinfo). Currently, it is a caller's duty to make sure the path is prepared. I am thinking into moving this functionality into the library, because

1. if the path is already prepared, this won't affect performance much (it's a few stat(2) syscalls);
2. if the path is not prepared and is resolved correctly, we avoid "false negative".

The implementation of 'resolve' is something like this:

        if realPath, err = filepath.Abs(path); err != nil {
                return false, fmt.Errorf("unable to get absolute path for %q: %w", path, err)
        }
        if realPath, err = filepath.EvalSymlinks(realPath); err != nil {
                return false, fmt.Errorf("failed to canonicalise path for %q: %w", path, err)
        }
        if _, err := os.Stat(realPath); err != nil {
                return false, fmt.Errorf(err, "failed to stat target %q of %q: %w", realPath, path, err)
        }

[kolyshkin]

NOTE that we already stat the target, so the addition of the above function won't help us to return early in case path is non-existent.

[kolyshkin]

Also, we can't do the same "resolve" for mount filters as there are no way to return an error from a filter, so the caller is still responsible.