Update security design documentation

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Move security stuff out of the object manager to FS

Under the new design, the only object type that would make use of the object
ACL/security mechanisms is file objects. Therefore, move the security related
bits currently in the object manager out to the file object code.

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>