inet_diag.c: added filter building code from socketstat (Alexey Kuznetsov, <kuznet@ms2.inr.ac.ru>)
             added filter options to create function
             added create function documentation

diag_filter.h: new header file
               added various filter condition constants
               added diag_filter structure

Signed-off-by: James Hulka <james.hulka@gmail.com>
Signed-off-by: Jiri Kastner <jkastner@redhat.com>