After copying "nm-vpn-plugin-old.*" to "nm-vpn-service-plugin.*",
rename the class and add it to the Makefile.

This will become the new VPN Service API for libnm 1.2. No changes
done yet except renaming of the classes and functions.

Rename the previous classes NMVpnPlugin(Old) to NMVpnServicePlugin
to have a distinct name from NMVpnEditorPlugin. Buth are plugins, but
with a different use.

https://bugzilla.gnome.org/show_bug.cgi?id=749951