WIP: introduced structure for tracking residual special files like ..last

..last, ..current, ..next, ..abend, and ..status are special files that only
exist on the fuse filesystem.  They persist after a file has been processed.
Therefore, a separate structure is needed apart from the instance structure,
since that gets freed when the file is closed.  Residual files are looked up
based on configuration variables.  This isn't perfect, since non-clustered
reads can occur in parallel.  To work around this, non-clustered users can
be differentiated by slightly different filesize options, eg:
cat /mnt/pcap/filesize=2048p/eth0.pcap.1234
cat /mnt/pcap/filesize=2000p/eth0.pcap.1235
There are still some bugs when using path-based options... sloppy me.