merged enum_net fix from RC_0_16

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9777 a83610d8-ad2a-0410-a6ab-fc0612d85776
make ignore_non_routers more forgiving in the case there are no UPnP devices at a known router. Should improve UPnP compatibility.

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9778 a83610d8-ad2a-0410-a6ab-fc0612d85776
merged additional unit test from RC_0_16

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9780 a83610d8-ad2a-0410-a6ab-fc0612d85776