Add ALTER TABLE ... ALTER COLUMN ... SET STATISTICS DISTINCT

doc/src/sgml/catalogs.sgml

doc/src/sgml/ref/alter_table.sgml

doc/src/sgml/ref/analyze.sgml

src/backend/access/common/tupdesc.c

src/backend/bootstrap/bootstrap.c

src/backend/catalog/heap.c

src/backend/catalog/index.c

src/backend/commands/analyze.c

src/backend/commands/tablecmds.c

src/backend/parser/gram.y

src/bin/pg_dump/pg_dump.c

src/bin/pg_dump/pg_dump.h

src/bin/psql/tab-complete.c

src/include/catalog/catversion.h

src/include/catalog/pg_attribute.h

src/include/catalog/pg_class.h

src/include/nodes/parsenodes.h