Extend EXPLAIN to provide hint about whether FILESORT will likely fit in memory or write to disk

When developers run EXPLAIN to find out if the query will be okay to run or not, the "filesort" in "extra info" is not clear about whether that is likely to be an in-buffer sort, or if it is a spill-to-disk sort. Perhaps (based on the row size, estimated rows involved, buffer size) a good guess can be added to the explain output.