Specify "client" per service, as documented in man

Looking at https://www.stunnel.org/static/stunnel.html shows that "client" (yes/no)
is actually a service configuration attirbute, not a global.

For backward-compatibility with previous versions, I left default[:stunnel][:client_mode]
but that only sets a "default" policy

It is now possible (and recommended) to specify the client mode in the provider definition
allowing multiple, different stunnels on the same node

```
stunnel_connection 'random_service' do
  accept node[:random_service][:tunnel_port]
  connect node[:random_service][:port]
  notifies :restart, 'service[stunnel]'
  client false # or true, if unspecified node[:stunnel][:client_mode] is used
end
```