Added what I think is full support for Null column values in the tables, partially...

Added what I think is full support for Null column values in the tables, partially based on submissions by Roger Gammans
Added a parameters to wxDbTable::Open() to indicate whether Open() should check for table privileges or not.
wxDbTable::ClearMemberVar() added to set a single column to an initialized value
::ClearMemberVar() and ::ClearMemberVars() can now optionally set a column to be NULL via ::SetColNull()
::SetColNull() added as the primary function name for setting a column to be a NULL value, and ::SetNull() is deprecated
::SetColNull() now has a param to select whether a columns value is NULL or not.
::SetColNull() will now clear the column value in memory when setting the column to be NULL.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@8767 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775