Improve generated column names for cases involving sub-SELECTs.

We'll now use "exists" for EXISTS(SELECT ...), "array" for ARRAY(SELECT
...), or the sub-select's own result column name for a simple expression
sub-select.  Previously, you usually got "?column?" in such cases.

Marti Raudsepp, reviewed by Kyotaro Horiugchi

doc/src/sgml/ref/select.sgml

@@ -758,8 +758,9 @@ UNBOUNDED FOLLOWING
     If you do not specify a column name, a name is chosen automatically
     by <productname>PostgreSQL</productname>.  If the column's expression
     is a simple column reference then the chosen name is the same as that
-    column's name; in more complex cases a generated name looking like
-    <literal>?column<replaceable>N</>?</literal> is usually chosen.
+    column's name.  In more complex cases a function or type name may be
+    used, or the system may fall back on a generated name such as
+    <literal>?column?</literal>.
    </para>
 
    <para>

doc/src/sgml/syntax.sgml

@@ -2109,9 +2109,9 @@ SELECT ARRAY[]::integer[];
    bracketed) subquery. For example:
 <programlisting>
 SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
-                          ?column?
--------------------------------------------------------------
- {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31}
+                                 array
+-----------------------------------------------------------------------
+ {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31,2412,2413}
 (1 row)
 </programlisting>
    The subquery must return a single column. The resulting