doc: change comment style

and move nlmsg_batch_reset_buffer() to mmap file
package: make functions private
attr: add put_check variant

raw one and Ptr suffix
attr: validate function retuns only error

without int
doc: describe nlmsghdr and nlattr

add package.go for it.
doc: update for godoc

and add original COPYING
example: update

SocketOpen() to NewSocket(),
NlmsgBatchStart() to NewNlmsgBatch()
and attr validation
doc: update README.md