[raster] replace "srs" parameter of ST_AsGDALRaster function with "srid"

[dustymugs]

This is to replace the "srs" function parameter ST_AsGDALRaster and any wrapper functions that make use of that parameter to "srid". In doing so, this will make these functions consistent to other functions involving spatial reference systems, such as ST_Transform.

Also, this will eliminate ST_srtext as a user-function and instead transform it to be a non-user function _ST_srtext. Another side affect of this change is the reduction of function variations for ST_AsGDALRaster and ST_AsTiff.

The behavior of the "srid" parameter is as follows:

• if a valid srid is provided, the underlying C function will fetch the associated srtext

• if the "srid" is NULL, the the underlying C function will fetch the srtext of the raster being coverted

• if the "srid" is -1, the outputted GDAL raster will not have any spatial reference system embedded (assuming the format permits it)

[dustymugs]

Changes made in r7346.