Don't generate FTN message-IDs for messages imported via FTN that are missing a

message-ID (e.g. when exporting from SBBSecho).
This addresses compliance with this [editorialized] clause in FTS-9:
     No system
     should ever add an MSGID and/or REPLY to,  or modify an existing
     MSGID / REPLY contained in,  a message not originating on that [FTN]
     system.

Messages gated from other networks (technically coming from another system,
but originating into an FTN from this system) will still have an FTN Message-ID
generated/added.

Since SBBSecho normally tosses to downlinks directly from packets, this adding
of generated Message-IDs would no normally occur. However, if a downlink
rescanned an area, any messages missing Message-IDs would get them generated
automatically and they would appear to have originating on the local system.
This was never the intention, so this is just a long standing but infrequently
observed (and never reported) bug.