Allow SQL:2008 syntax ALTER TABLE ... ALTER COLUMN ... SET DATA TYPE

doc/src/sgml/ref/alter_table.sgml

src/backend/catalog/sql_features.txt

src/backend/parser/gram.y

src/backend/parser/keywords.c

src/interfaces/ecpg/preproc/ecpg_keywords.c

src/interfaces/ecpg/preproc/preproc.y

src/interfaces/ecpg/test/expected/sql-parser.c

src/interfaces/ecpg/test/expected/sql-parser.stderr

src/interfaces/ecpg/test/sql/parser.pgc