When replying to PING netmails, use the destination addr as the origaddr

As tested and reported in FIDONEWS by Michiel van der Vlist, 2:280/5555, SBBSecho would use the "best match" FidoNet AKA for the originating address when replying to PING netmail messages and not necessarily the original destination address of the ping request. For systems that have multiple addresses (AKAs) that could be considered appropriate originating addresses for the requesting node address (e.g. multiple addresses in the same zone or zone/net), this could cause a confusion for the PING requester.

The create_netmail() function now accepts an optional source (orig) address parameter and the PING response logic passes the netmail's destination address for the reply message's originating (source) address.

I noticed that AreaMgr responses also follow the same logic as PING responses (just use the best-fit AKA, not necessarily the same address as the original request's destination address) - but I did not choose to address that "issue" at this time.