Indexing removable volumes in the main index

As of version 1.25.2, Recoll has a simple way to ensure that the index data for an absent volume will not be purged: the volume mount point must be a member of the topdirs list, and the mount directory must be empty (when the volume is not mounted). If recollindex finds that one of the topdirs is empty when starting up, any existing data for the tree will be preserved by the indexing pass (no purge for this area).