Fix pg_get_indexdef()'s behavior for included index columns.

The multi-argument form of pg_get_indexdef() failed to print anything when
asked to print a single index column that is an included column rather than
a key column.  This seems an unintentional result of someone having tried
to take a short-cut and use the attrsOnly flag for two different purposes.
To fix, split said flag into two flags, attrsOnly which suppresses
non-attribute info, and keysOnly which suppresses included columns.
Add a test case using psql's \d command, which relies on that function.

(It's mighty tempting at this point to replace pg_get_indexdef_worker's
mess of boolean flag arguments with a single bitmask-of-flags argument,
which would allow making the call sites much more self-documenting.
But I refrained for the moment.)

Discussion: https://postgr.es/m/21724.1531943735@sss.pgh.pa.us

src/test/regress/expected/index_including.out

@@ -19,6 +19,16 @@ WHERE i.indrelid = 'tbl_include_reg'::regclass ORDER BY c.relname;
  CREATE INDEX tbl_include_reg_idx ON public.tbl_include_reg USING btree (c1, c2) INCLUDE (c3, c4)
 (2 rows)
 
+\d tbl_include_reg_idx
+Index "public.tbl_include_reg_idx"
+ Column |  Type   | Definition 
+--------+---------+------------
+ c1     | integer | c1
+ c2     | integer | c2
+ c3     | integer | c3
+ c4     | box     | c4
+btree, for table "public.tbl_include_reg"
+
 -- Unique index and unique constraint
 CREATE TABLE tbl_include_unique1 (c1 int, c2 int, c3 int, c4 box);
 INSERT INTO tbl_include_unique1 SELECT x, 2*x, 3*x, box('4,4,4,4') FROM generate_series(1,10) AS x;

src/test/regress/sql/index_including.sql

@@ -14,6 +14,7 @@ CREATE INDEX ON tbl_include_reg (c1, c2) INCLUDE (c1, c3);
 SELECT pg_get_indexdef(i.indexrelid)
 FROM pg_index i JOIN pg_class c ON i.indexrelid = c.oid
 WHERE i.indrelid = 'tbl_include_reg'::regclass ORDER BY c.relname;
+\d tbl_include_reg_idx
 
 -- Unique index and unique constraint
 CREATE TABLE tbl_include_unique1 (c1 int, c2 int, c3 int, c4 box);