network: use VPN plugin paths as specified in the .name files

Since LIBDIR of gnome-control-center and the VPN plugins aren't
necessarily the same, use the paths as specified by the VPN plugins and
only reconstruct the path if it is not absolute or we fail to load the
plugin.

Patch from Debian package, by Michael Biebl <biebl@debian.org>

https://bugzilla.gnome.org/show_bug.cgi?id=764509