NetworkManager (the daemon) has no defined working directory, so
it can only handle absolute path names. This is in general and also for
the LoadConnections() D-Bus call.

That means, nmcli should make relative paths absolute.

We don't use g_canonicalize_filename() because that also cleans up
double slash and "/./". I don't think we should do that in this case, we
should only prepend $PWD to make the path absolute.

#794