Use proper logging macros.

nm_vpn_connection_activate() does have it's use, because it's the
gatekeeper to avoid activating multiple connection for old plugins.

nm_vpn_connection_deactivate() doesn't seem any useful.

only add vpn plugins that have a program set in the name-file

Plugins also have a use in client side only, so it is conceivable
to have a name-file without program name set. Or it might be just
a bug in the name-file. Anyway, ignore them.

Add assertions.

Don't require @error argument.

A separate instance of the support plugin is spawned for each connection with
a different bus name. The bus name is passed via --bus-name <name> argument.
Plugins that support the feature indicate it with
support-multiple-connections=true key in the [VPN Connection] section.

The bus name is currently generated by adding a .<connection.uuid> to the VPN
service name. It's guarranteed unique, but if it proves to be too long or ugly
it can easily be replaced with something more meaningful (such as the same number
as is used for connection's DBus name).

NMVpnService has been removed and folded into NMVpnConnection. A
NMVpnConnection will spawn a service plugin instance whenever it is activated
and notices the bus name it needs is not provided.

The NMVpnManager no longer needs to keep track of the connections in use apart
for compatibility purposes with plugins that don't support the feature.