vfs: sys_link() initial version

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tst-fs-link: Adding an extra check.

Adding a new test to see if the content from one link would be
the same in another.

Signed-off-by: Raphael S. Carvalho <raphael.scarv@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Revert "vfs: sys_link() initial version"

This reverts commit 7f2148dca1eb50849c577f2d0234b8696c8c1624.
Adding link system call.

This patch adds the link system call.
It's important to realize that links will only work if the underlying
file system supports it, otherwise EPERM will be returned.
ZFS is currently the unique file system that supports it.
By the way, some bsd files were modified according to the actual needs,
e.g. solve dependencies.

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