Rename libnm's NMVpnPlugin to NMVpnPluginOld, in preparation for
having a new-and-improved NMVpnPlugin in NM 1.2. Also remove it from
NM-1.0.gir.

Make nm-vpn-plugin-old.h be separately includable, since it's not
included from NetworkManager.h, and we probably don't want it to be.

Remove NMVpnPlugin, NMVpnPluginUiInterface, and nm-vpn-plugin-utils
from the docs, since they're basically undocumented anyway.