Improve updatability checking for views and foreign tables.

Extend the FDW API (which we already changed for 9.3) so that an FDW can
report whether specific foreign tables are insertable/updatable/deletable.
The default assumption continues to be that they're updatable if the
relevant executor callback function is supplied by the FDW, but finer
granularity is now possible.  As a test case, add an "updatable" option to
contrib/postgres_fdw.

This patch also fixes the information_schema views, which previously did
not think that foreign tables were ever updatable, and fixes
view_is_auto_updatable() so that a view on a foreign table can be
auto-updatable.

initdb forced due to changes in information_schema views and the functions
they rely on.  This is a bit unfortunate to do post-beta1, but if we don't
change this now then we'll have another API break for FDWs when we do
change it.

Dean Rasheed, somewhat editorialized on by Tom Lane

doc/src/sgml/postgres-fdw.sgml

@@ -254,6 +254,43 @@
    </para>
 
   </sect3>
+
+  <sect3>
+   <title>Updatability Options</title>
+
+   <para>
+    By default all foreign tables using <filename>postgres_fdw</> are assumed
+    to be updatable.  This may be overridden using the following option:
+   </para>
+
+   <variablelist>
+
+    <varlistentry>
+     <term><literal>updatable</literal></term>
+     <listitem>
+      <para>
+       This option controls whether <filename>postgres_fdw</> allows foreign
+       tables to be modified using <command>INSERT</>, <command>UPDATE</> and
+       <command>DELETE</> commands.  It can be specified for a foreign table
+       or a foreign server.  A table-level option overrides a server-level
+       option.
+       The default is <literal>true</>.
+      </para>
+
+      <para>
+       Of course, if the remote table is not in fact updatable, an error
+       would occur anyway.  Use of this option primarily allows the error to
+       be thrown locally without querying the remote server.  Note however
+       that the <literal>information_schema</> views will report a
+       <filename>postgres_fdw</> foreign table to be updatable (or not)
+       according to the setting of this option, without any check of the
+       remote server.
+      </para>
+     </listitem>
+    </varlistentry>
+
+   </variablelist>
+  </sect3>
  </sect2>
 
  <sect2>