Reset require_fresh_start_ after buffer has been viewed/exported

Reset require_fresh_start_ after buffer has been viewed/exported

The LFUN buffer-reset-export states to start afresh the export for the
next run, but in fact, the boolean was never reset, so that this was
done for all subsequent runs unless the buffer had been closed and
reopened.

Now the lfun does what it states and only acts on the next pass.

(cherry picked from commit e1cb15ee5d9377cef6e3683a6d0bb68eb9b379c1)