Bug fix for net type when forwarding messages to internet email or fidonet netmail

When forwarding a message, it now correctly sets sets the to_net_type property in the message header to FidoNet or internet for those types of message destinations - at least for internet & FTN addresses. I wasn't sure where I might set it to NET_POSTLINK, NET_WWIV, or NET_MHS, as I haven't found an example yet of what those addresses look like.