Use GRASS_GISBASE for GDAL GRASS driver

[Even Rouault]

This ticket is to track implementation of William's suggestion :

The configured GRASS prefix is already compiled into the OGR GRASS  
driver as GRASS_GISBASE, why not do the same for the GDAL GRASS  
driver?  (I thought it was, but GRASS_GISBASE only turns up in  
ogrgrassdatasource.cpp).

[Even Rouault]

Use GRASS_GISBASE for GDAL GRASS driver instead of hard-coded path in trunk (r15901)

Minor fix in trunk (r15902) for OGR GRASS driver to prevent memory leak and potential buffer overflow

[kyngchaos]

Tested patch on 1.6, autotest grass_2 now succeeds.

thanks