Strip the "in-transit" attribute flag from imported packed-messages

Log a warning message when either the "local" or "in-transit" flag has been
stripped from ("Sanitized") imported packed-messages.

Keyop reported a chksmb warning on a FidoNet message that contained the
'in-transit' attribute:
<Keyop> Message flagged as 'In Transit'     (?): 1
<Keyop> fidonet_fidosoft Message Base has Errors!

The message in question appears just to be a corrupted echomail msg coming
from HPT:
Sender           '56:43 '
To               Tommi Koivula
Subject          test
X-FTN-AREA       FIDOSOFT.HUSKY
X-FTN-TID        hpt/os2-wc 1.9.0-cur 2021-03-14

SBBSecho didn't use to convert/import the "in-transit" flag on imported
netmail or echomail messages, ... until commit 446ab0ef, which
was needed to support the TickFix robot. So now, let's just sanitize the
packed-message attributes a little sooner in the import process and log a
warning when such sanitization has taken place.