NAD27 expands to use towgs84 instead of grid shift files

[warmerdam]

For reasons that are not clear, the expansion of "NAD27" in OGRSpatialReference::SetWellKnownGeogCS() has include a TOWGS84[] approximation in the WKT for a very long time.

With the introduction of r19903 the translation of the WKT to PROJ.4 format uses a +towgs84 parameter instead of the +nadgrids directive (previously implicit from +datum).

On a related note the current expansion of EPSG:4326 (NAD27) uses a towgs84 value from the datum_shift.csv.