Beautify files.h

 - add argument names
 - align members
 - add extern kw
 - group struct decl. on top
 - make file_desc_ops::type being unsigned
   integer similar to fdtype_ops::type

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>