Add SMTP interface

I added a SMTP interface to make it possible to send messages with MUAs that can't call sendmail (like thunderbird).
It doesn't support any kind of authentication or encryption. That's why I forced it to use localhost.
The SMTP interface just greps the mails, adds BCC recipients to the metadata and calls ntb_ipc_client_send_command(), similar to ntb_sendmail().

I wasn't able to send you a message to BM-orkCbppXWSqPpAxnz6jnfTZ2djb5pJKDb with all that spam, else I would have ask you before just sending this pull request.