functions using cb: returns error too

not only return MNL_CB_... but also error
move functions to go_receiver

I think it's fit now.
add lacking functions

I've foggoten mnl_nlmsg_size(), mnl_nlmsg_payload_len().
in addition:
  add functions getting payload as []byte
  use C.GoBytes in BatchHead()
    I refered in ``Turning C arrays into Go slices'' http://code.google.com/p/go-wiki/wiki/cgo
    but C.GoBytes() was introduced after that?
reflect recent changes

since 6a1628e5
introduce construct helper

seems idiomatic from in my very narrow view
socket: return nsent

I think it's needed now.
examples: follow the current

using PutNewNlmsghdr() and SendNlmsghdr()
doc: you must execuse my awful English

...sorry,