fcl-base/dbase: 
* when tdbf "auto upgrades" foxpro to visual foxpro, write the missing backlink info
* updated tablelevel test that deals with FoxPro=>VFP "auto upgrade"
* default block length for (visual) foxpro files is 64, not 512
* match (Visual)FoxPro empty characters in memo field


git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@24206 3ad0048d-3df7-0310-abae-a5850022a9f2