All: Dataflash log file size rotation

Some users run their vehicles for a VERY long time. In the case of boat users they often run 24 hours a day over several days. At the moment his generates one monolithic huge log file. First question is what happens when the SDCard becomes full and its only got 1 file on it and its the file currently being written to? Ideally at a certain point we would split the log file into separate files of say 1Gig each for ease of handling. There are a few hurdles to overcome.
All the self describing information, parameters etc are gathered at the beginning of the log file so we either have to copy that stuff to the new log file as its created and treat each log file as self contained OR assume that subsequent log files are part of the first one and we need to stitch them back together after download or have the log file processing tools be able to handle multiple files.
If we do split into multiple files we still need to decide what happens when the SDCard becomes full.

No files, tests, or entry points are named. Begin by locating Dataflash logging and SD-card handling, then determine how rotation, metadata copying or stitching, and a full card should behave; done means the design is agreed and verified for long-running logs.

Written by the indexing model from the issue text.