Add/use new FTN "BBSID" control paragraph (kludge line)

Advertise the system's QWK-ID (a.k.a. BBS ID) in exported echomail messages. This will allow correlation of avatars that were imported via SYNCDATA carried via QWKnet (e.g. DOVE-Net) with messages imported from those same BBSes via FTN.

The alternative was to have SYNCDATA Avatar messages include all the AKAs of each BBS (in the body text), but that was looking like a rather complicated solution. This approach (the BBSID kludge) seems a much simpler solution.

I still need to implement the JS side of this solution however (for avatar lookups using the FTN BBSID), but this was the first important step.