Use symbolic references for pg_language OIDs in the bootstrap data.

This patch teaches genbki.pl to replace pg_language names by OIDs
in much the same way as it already does for pg_am names etc, and
converts pg_proc.dat to use such symbolic references in the prolang
column.

Aside from getting rid of a few more magic numbers in the initial
catalog data, this means that Gen_fmgrtab.pl no longer needs to read
pg_language.dat, since it doesn't have to know the OID of the "internal"
language; now it's just looking for the string "internal".

No need for a catversion bump, since the contents of postgres.bki
don't actually change at all.

John Naylor

Discussion: https://postgr.es/m/CAJVSVGWtUqxpfAaxS88vEGvi+jKzWZb2EStu5io-UPc4p9rSJg@mail.gmail.com

doc/src/sgml/bki.sgml

@@ -409,8 +409,9 @@
     that's error-prone and hard to understand, so for frequently-referenced
     catalogs, <filename>genbki.pl</filename> provides mechanisms to write
     symbolic references instead.  Currently this is possible for references
-    to access methods, functions, operators, opclasses, opfamilies, and
-    types.  The rules are as follows:
+    to access methods, functions, languages,
+    operators, opclasses, opfamilies, and types.
+    The rules are as follows:
    </para>
 
    <itemizedlist>
@@ -421,6 +422,7 @@
       by attaching <literal>BKI_LOOKUP(<replaceable>lookuprule</replaceable>)</literal>
       to the column's definition, where <replaceable>lookuprule</replaceable>
       is <literal>pg_am</literal>, <literal>pg_proc</literal>,
+      <literal>pg_language</literal>,
       <literal>pg_operator</literal>, <literal>pg_opclass</literal>,
       <literal>pg_opfamily</literal>, or <literal>pg_type</literal>.
       <literal>BKI_LOOKUP</literal> can be attached to columns of