LP#1949243: tweak FTX and LIN in EDI messages

LP#1949243: tweak FTX and LIN in EDI messages

There are a couple of errors in EDIWriter's output:

1. The FTX line is missing a field. We produce FTX+LIN+1+First Nations
but should be producing FTX+LIN+1++First Nations (note the extra +
sign).

2. We are enumerating lineitems using the internal lineitem ID (e.g.
LIN+523964), but they should be enumerated sequentially starting at 1
(e.g. LIN+1).

This commit fixes both errors.

Signed-off-by: Jeff Davis <jdavis@sitka.bclibraries.ca>