zdtm: add a test case for netlink sockets

* create two bound socket (for unicase and broadcase messages),
  a connected socket, an unbound and unconnected socket.
protobuf: add image for netlink sockets
crtools: add a function for showing netlink sockets
crtools: add constants for a netlink image
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