add a raster_upgrade_20_minor script

[robe]

This will allow users to upgrade from now to release time assuming we don't make any structural changes to the raster type or define new operators.

First part is the sam eas what would be needed for uninstall -- drop obsolete functions.

[robe]

I have a preliminary one building at r7930 and works fine for current version.

However it doesn't upgrade one of my older dbs because I'm still missing a drop in the rtpostgis_drop.sql.in.c of a function that changed output parameters.

Guys -- do me a favor, moving forward - If you are going to rename, change the arguments of, or output type of a function, please add the older version to the rtpostgis_drop.sql.inc.c file.

The way upgrade script works -- it builds by first dropping obsolete functions (whcih I can't tell from teh rtpostgis.sql.inc.c

and then sources the rtpostgis.sql.inc.c to CREATE OR REPLACE functions.

Functions that have changed signature or outputs need to be dropped first otherwise the CREATE OR REPLACE compalins the signature has changed etc.

I'll close this out once I've found the remaining offending obsolete signatures.

[robe]

This I have done too (forgot which svn rev) -- forgot to close it out, but seems to be working pretty well for me with the rtpostgis_drop.sql.in.c we have