Support INOUT arguments in procedures

In a top-level CALL, the values of INOUT arguments will be returned as a
result row.  In PL/pgSQL, the values are assigned back to the input
arguments.  In other languages, the same convention as for return a
record from a function is used.  That does not require any code changes
in the PL implementations.

Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>

doc/src/sgml/plperl.sgml

@@ -278,6 +278,20 @@ SELECT * FROM perl_row();
    hash will be returned as null values.
   </para>
 
+  <para>
+   Similarly, output arguments of procedures can be returned as a hash
+   reference:
+
+<programlisting>
+CREATE PROCEDURE perl_triple(INOUT a integer, INOUT b integer) AS $$
+    my ($a, $b) = @_;
+    return {a =&gt; $a * 3, b =&gt; $b * 3};
+$$ LANGUAGE plperl;
+
+CALL perl_triple(5, 10);
+</programlisting>
+  </para>
+
   <para>
     PL/Perl functions can also return sets of either scalar or
     composite types.  Usually you'll want to return rows one at a