[Lazarus] TDBF: Initial Visual FoxPro support

[Reinier Olislagers]

Hi FPC & Lazarus lists,

I've been working on the tdbf code.

It had rudimentary support for both Visual Foxpro and older Foxpro file
formats, but mixed them up a bit.

I split out the distinction so you can specify either foxpro tablelevel
(25) or Visual Foxpro (30). Just like the original code and the way
Visual Foxpro works though, Foxpro tables can be modified by the
application programmer to include VFP only features; in this case the
table is automatically upgraded to VFP level (30).

Initial support for V (varchar), Q (varbinary), 0 (bytes) fields has
been added, as well as support for nullable fields. Thanks to Ludo
Brands for help with what bits and bytes to shift ;)

There are likely still rough edges - e.g. codepage translation for V
fields is probably missing; I'll continue to run and expand the
dbtestfraemwork test suite to catch and improve those.

Apart from the (V)FP improvements I committed today, I've recently fixed
done some other work - please see the svn commit log for details.

Next on the list is getting dbaseIII memo support correct for the
ultimate back to the '80s feeling ;)
Also, Visual Foxpro tables now show the system/hidden field _NULLFLAGS
used to store null... ehrm flags; obviously this should be hidden from
the programmer.

Suggestions welcome, and mantis patches/bug reports as well!

Thanks,
Reinier