ogr2ogr bug with mapinfo

[warmerdam]

The command:

 ogr2ogr -f "MapInfo File" out.mif llpsamp1.ntf 

creates an empty out.mif/out.mid, presumably because there is not matching
feature specification for any of the layers in llpsamp1.ntf, while the following
works fine writing out a file for each feature type:

 ogr2ogr -f "MapInfo File" out llpsamp1.ntf

While it may be OK to not write anything, the translation should report some
sort of error indicator!

[warmerdam]

I have established that the various feature types are being written out to
other .mid/mif files based on the feature type.  This likely shouldn't be
when writing out a single file. 

[warmerdam]

I have modified mitab_ogr_datasource and mitab_ogr_driver in the upstream code 
to enforce that single file datasets only have a single layer.  Also, that 
the first CreateLayer() call will use it but that no other ones will.  And
finally an indirectly related change so that existing datasources can be 
"Created" as datasources. 

This brings the MITAB driver pretty much into line with the behaviour of the
shapefile driver.