Support Directory Deletion

After we can create directories - we can try to delete them, too, with stamfs_iop_rmdir():

• Make sure the directory is empty (it's up to US, not to the VFS. hence - we could have changed this semantics if we wanted to).
• Unlink the directory (see next slide).
• Decrease the parent's link count - the child's ".." no longer points to it.
• Decrease the child's link count - the child's "." no longer points at itself.
• Do not actually delete the directory's inode - the VFS will do it once it sees the directory's link-count dropped to 0.

Originally written by guy keren