crtools: add netlink_diag.h

This header is taken from kernel sources.
dump: use socket diaf for collecting netlink sockets
sockets: dump netlink sockets

All info about bound sockets are got via socket diag interface.
All connected sockets are automatically bound.
For other sockets only protocol must be dumped, which is got
with help getsockopt.

A netlink sockets with pending data are not supported yet and
probably will not be supported in a near future.
crtools: restore netlink sockets