Ported commit 45b1a34627318967e2ca23f3bcebec1d67525ded from castor/master

CASTOR-4889: tapeserverd's end of session summary could indicate the
files still open

Fixed.

Add reports for still open files in the "Tape session finished"
log message to indicate problematic disk servers. It should be
only visible in case of kill of the data transfer session.