allow linking against libnl 2.0

allow linking against libnl 2.0

This converts iw to use libnl-2, and adds compatibility with libnl-1.
There is not currently a good way to detect the libnl version during
compilation, as the versioning in the netlink/version.h is defined as a
string "2.0" rather than a major and a minor number, so we must detect
it in the Makefile.

Signed-off-by: Pat Erley <pat-lkml@erley.org>