HPCC-9546 ecl-roxie-unused-files command

Adds an unused-files command to ecl-roxie which lists files found on
a given roxie cluster in DFS, but not currently in use on that
cluster.  --check-packagemaps option excludes files referenced in an
active packagemap.

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>