mvc_export_prepare: write correct result header
mvc_export_prepare: write correct result header

Don't lie to the client about the number of columns returned, JDBC gets
very upset about that.  Don't write inconsistent headers, JDBC refuses
to accept that.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb