Make DatumGetInetP() unpack inet datums with a 1-byte header, and add

a new macro, DatumGetInetPP(), that does not. This brings these macros
in line with other DatumGet*P() macros.

Backpatch to 8.3, where 1-byte header varlenas were introduced.

src/backend/utils/adt/network.c

@@ -907,7 +907,7 @@ convert_network_to_scalar(Datum value, Oid typid)
 		case INETOID:
 		case CIDROID:
 			{
-				inet	   *ip = DatumGetInetP(value);
+				inet	   *ip = DatumGetInetPP(value);
 				int			len;
 				double		res;
 				int			i;