The commit adds the newly developed 'File-Level Document-Version History' feature (fldv-history for short in the code). The work targets the 'archives' directory area. The core idea is to have older versions of a archives' document folder contained inside the latest generated version. To achieve this, inside an archives' document folder there can now be a '_fldv_history' folder, and inside that have sub-folders conforming the patter 'nminus-1', 'nminus-2', ... These are the older versions of the archives' document. The filenames are literally as just typed: nminus-1 contains the most recent stored version of the document; nminus-2 (if exists) is the second most recent version, and so on. When import.pl is run with -incremental and -keepold then any existing documents that need to be re-processed will trigger the formation of _fldv_history/nminus-1, storing the previous version in it. If import.pl -incremental -keepold is run again and the doc has changed again, then nminus-1 is moved to nminus-2, and a new nminus-1 is generated. With the addition of this new feature, there is a use-case of running -keepold without -incremental (and for it to be 'addonly'). To be clear, the 'onlyadd' functionality still works, however the code now does extra work to ensure that any existing documents in archives get the file-level document-version history treatment as well. This allows for a collection building to manually add content into import (even choose to leave existing content previously processed there if they want), and when import.pl -keepold is next run then a 'collection-wide' file-level document-version history of existing documents is triggered in 'archives'. The idea of a 'colletion-wide' (global) document history mechanism could be a useful way for a user to manage their collection. Hardlinking is used throughout the new code, so that occurs on the file system is not particularly expensive, although the overall collection build takes longer than 'onlyadd' as it does reprocess off the existing documents again. In the case of a user running import.pl -keepold and realizing this was in fact a mistake, there a new minus option '-replaceold'. This works in much the same way as 'keepold' only when it comes to the file-level document-version history feature, it does not add in yet another stored document version, rather it replaces the one previously stored at 'nminus-1' with this one, effectively undoing the 'mistake' of the previous build.
