feat(mount): add data field to MountSpec

[noeljackson]

Summary

Add an optional data: Option<String> field to MountSpec and pass it through to the mount(2) syscall. This enables mount options that require the data argument, such as size=64m for tmpfs or filesystem-specific options.

Changes

1. config.rs: Add data: Option<String> field to MountSpec struct.

2. mount.rs:

   • Pass the data field to mount(2) as the 5th argument (was always ptr::null()).
   • Improve the error message on mount failure to include source, target, fstype, bind flag, and flags hex value. The previous error was just unable to mount with no context.
   • Fix create_mountpoint for bind mounts: when the source is a non-directory (e.g., a device node or regular file), create an empty file at the target instead of a directory. Bind-mounting a file onto a directory fails with EINVAL.