Fix checking of query type in plpgsql's RETURN QUERY command.

Prior to v14, we insisted that the query in RETURN QUERY be of a type
that returns tuples.  (For instance, INSERT RETURNING was allowed,
but not plain INSERT.)  That happened indirectly because we opened a
cursor for the query, so spi.c checked SPI_is_cursor_plan().  As a
consequence, the error message wasn't terribly on-point, but at least
it was there.

Commit 2f48ede lost this detail.  Instead, plain RETURN QUERY
insisted that the query be a SELECT (by checking for SPI_OK_SELECT)
while RETURN QUERY EXECUTE failed to check the query type at all.
Neither of these changes was intended.

The only convenient place to check this in the EXECUTE case is inside
_SPI_execute_plan, because we haven't done parse analysis until then.
So we need to pass down a flag saying whether to enforce that the
query returns tuples.  Fortunately, we can squeeze another boolean
into struct SPIExecuteOptions without an ABI break, since there's
padding space there.  (It's unlikely that any extensions would
already be using this new struct, but preserving ABI in v14 seems
like a smart idea anyway.)

Within spi.c, it seemed like _SPI_execute_plan's parameter list
was already ridiculously long, and I didn't want to make it longer.
So I thought of passing SPIExecuteOptions down as-is, allowing that
parameter list to become much shorter.  This makes the patch a bit
more invasive than it might otherwise be, but it's all internal to
spi.c, so that seems fine.

Per report from Marc Bachmann.  Back-patch to v14 where the
faulty code came in.

Discussion: https://postgr.es/m/1F2F75F0-27DF-406F-848D-8B50C7EEF06A@gmail.com

Author: tglsfdc

doc/src/sgml/spi.sgml

@@ -739,6 +739,17 @@ int SPI_execute_extended(const char *<parameter>command</parameter>,
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>bool <parameter>must_return_tuples</parameter></literal></term>
+    <listitem>
+     <para>
+      if <literal>true</literal>, raise error if the query is not of a kind
+      that returns tuples (this does not forbid the case where it happens to
+      return zero tuples)
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>uint64 <parameter>tcount</parameter></literal></term>
     <listitem>
@@ -1869,6 +1880,17 @@ int SPI_execute_plan_extended(SPIPlanPtr <parameter>plan</parameter>,
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>bool <parameter>must_return_tuples</parameter></literal></term>
+    <listitem>
+     <para>
+      if <literal>true</literal>, raise error if the query is not of a kind
+      that returns tuples (this does not forbid the case where it happens to
+      return zero tuples)
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><literal>uint64 <parameter>tcount</parameter></literal></term>
     <listitem>