WL#5020: improve checksum returned by CHECKSUM TABLE

Here are some problems with the current CHECKSUM TABLE:
- http://bugs.mysql.com/bug.php?id=39474
"checksum table reports equal checksums when data is different"
- a MyISAM table with variable-size records and a MyISAM table with fixed-size
records have a different checksum though data is identical (testcase attached to
this WL).
5.1 and 6.0, though they report different checksums (because code was changed in
6.0), both have those problems.