In hindsight, the NMVpnPluginOld should never have made public for
nm-1-0 as there are no users and we don't want to support this API.

For now, just deprecate it.

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

Files are yet unchanged, only copy them to get a nicer history.

  /bin/cp libnm/nm-vpn-plugin-old.c libnm/nm-vpn-service-plugin.c
  /bin/cp libnm/nm-vpn-plugin-old.h libnm/nm-vpn-service-plugin.h

These files are not used. Just adjust them as proof-of-concept.
Disable them again.

vpn-helper.c is currently not yet used. It was added for future
VPN support for nmtui. Refactor it to make use of the new vpn
helper functions in libnm.

Use NMVpnPluginInfo to load the plugins in NMVpnManager.

This has the advantage of reusing the code from libnm
to use the same approach to read the plugin config files.

Another advantage is that we now check the file permissions
of the config file.