Redo mount(2)/umount(2) ABI

Redo mount(2)/umount(2) ABI

- pass in file system type through mount(2), and return this type in
  statvfs structures as generated by [f]statvfs(2);
- align mount flags field with NetBSD's, splitting out service flags
  which are not to be passed to VFS;
- remove limitation of mount ABI to 16-byte labels, so that labels
  can be made larger in the future;
- introduce new m11 message union type for mount(2) as side effect.

Change-Id: I88b7710e297e00a5e4582ada5243d3d5c2801fd9