Fix libnl/libnl-3 logic in configure script

This patch causes the configure script to prefer libnl-3 over
libnl(1). The configure script will first check for libnl-3 and
libnl-genl-3. If both are found, use them. If not, check for
libnl(1). This is useful when building on systems that have both
libnl-3 and libnl(1) installed. It also fixes some redundant libraries
in LIBS.