This patch adds the link() system call.

The system call uses the filesystem specific VOP_LINK vnode operation to
do the actual work.  ZFS is currently the only available file system
that supports hard links and the other filesystems return EPERM.

Signed-off-by: Raphael S. Carvalho <raphael.scarv@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>