NetworkManager: detect libnm before looking for NetworkManager
NetworkManager: detect libnm before looking for NetworkManager

Prefer the newer libnm symbol and only fall back to the legacy NetworkManager
if not found

Fixes https://github.com/libproxy/libproxy/issues/26 - 2nd part

maxinbjohn/libproxy,libproxy/libproxy,horar/libproxy,horar/libproxy,horar/libproxy,libproxy/libproxy,libproxy/libproxy,libproxy/libproxy,maxinbjohn/libproxy,maxinbjohn/libproxy,libproxy/libproxy,horar/libproxy,libproxy/libproxy,maxinbjohn/libproxy,horar/libproxy,maxinbjohn/libproxy,maxinbjohn/libproxy,horar/libproxy,maxinbjohn/libproxy,libproxy/libproxy,horar/libproxy