api: change nlstruct class method name

sizeof() to csize()
pointer() to from_pointer()
module name: indicate internal use

http://stackoverflow.com/questions/3602110/python-private-module-in-a-package
comment: remove garbage
socket: setsockopt returns no value

only raise OSError in case of error
nlmsg: move put_new_header under Header class

made it static method in Header class.
And put_check() requires ctypes param as same as put()
package: select exporting

and add original COPYING
status: change to alpha