Code comments and tweaks for lost processing

Code comments and tweaks for lost processing

This commit is intended to do the following:

* Fix case where MISSING status is not retained for items
  being sent 'home' (as it was in previous versions)
* Be more explicit about when we unset the LOST status and when
  we do not in checkin_handle_lost()
* Prevent update of the copy in checkin_handle_lost() unless we
  actually change the status
* Restore log messages for special handling of LOST/MISSING
  checkins away from 'home'
* Provide additional code comments to clarify intended behavior

Note: Given the current restructure, MISSING item behavior is no
longer affected by 'lost_immediately_available' setting.  That
seemed more bug than feature, and can be implemented later
(perhaps as a separate setting) if required.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>