Added Blackboard restore using XSLT. Works with BB 5.5 and BB 6.0.

Added Blackboard restore using XSLT.  Works with BB 5.5 and BB 6.0.

Completely untested by me but it LOOKS like it should work.  It's quite
standalone, and is basically a single function call during the restore
precheck which looks in the backup to see if it's a BB file, then
converts it using XSLT.

Apart from a little tidyup and renaming, I added a check so that it
should just silently fail if XSLT functions aren't available

Many thanks to Ziba Scott <ziba@linuxbox.com> who wrote it!