Added preliminary DB2 support.

Added preliminary DB2 support.
Removed the #if EXPERIMENTAL_WXDB_FUNCTIONS blocks from around functions that will now be available in 2.4
Changed functions to return const char * rather than char * on certain functions
Added the wxDb::TablePrivileges() function
Added wxDb copy constructors and wxDbGetConnection() copy constructors which are able to copy the typing definitions of data types retrieved from the source wxDb instance to the new data connection instance to avoid the network/database overhead of querying the datasource for its data types once they are already known once.
SQL Server and (preliminary) DB2 support added
If compiling with Visual C++, the wxDbCreateDataSource() can now be enabled

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@8761 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775