Fix tablespace inheritance for partitioned rels

Commit ca4103025d left a few loose ends.  The most important one
(broken pg_dump output) is already fixed by virtue of commit
3b23552ad8, but some things remained:

* When ALTER TABLE rewrites tables, the indexes must remain in the
  tablespace they were originally in.  This didn't work because
  index recreation during ALTER TABLE runs manufactured SQL (yuck),
  which runs afoul of default_tablespace in competition with the parent
  relation tablespace.  To fix, reset default_tablespace to the empty
  string temporarily, and add the TABLESPACE clause as appropriate.

* Setting a partitioned rel's tablespace to the database default is
  confusing; if it worked, it would direct the partitions to that
  tablespace regardless of default_tablespace.  But in reality it does
  not work, and making it work is a larger project.  Therefore, throw
  an error when this condition is detected, to alert the unwary.

Add some docs and tests, too.

Author: Álvaro Herrera
Discussion: https://postgr.es/m/CAKJS1f_1c260nOt_vBJ067AZ3JXptXVRohDVMLEBmudX1YEx-A@mail.gmail.com

doc/src/sgml/ref/create_table.sgml

@@ -1265,9 +1265,9 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       <xref linkend="guc-default-tablespace"/> is consulted, or
       <xref linkend="guc-temp-tablespaces"/> if the table is temporary.  For
       partitioned tables, since no storage is required for the table itself,
-      the tablespace specified here only serves to mark the default tablespace
-      for any newly created partitions when no other tablespace is explicitly
-      specified.
+      the tablespace specified overrides <literal>default_tablespace</literal>
+      as the default tablespace to use for any newly created partitions when no
+      other tablespace is explicitly specified.
      </para>
     </listitem>
    </varlistentry>