generic: ACHTUNG: get_attr() semantics changed

Now `nla.get_attr()` return the first NLA with that name OR None if no
such attrs present. To get all NLA with that name, use `nla.get_attrs()`.

Rationale: usually, there is only one NLA with given name in NLA chain.
generic: get_size() class method

Returns calculated sum size of all fields (w/o NLA)
tests: ingress filters and sfq tests
examples: trace arbitrary command from command line