doc: add missing xreflabels to the main docs (not refs)

Add missing xreflabels for index types, geqo, libpq, spi, server-side
languages, ecpg, and vaacuumlo.

Backpatch-through: 9.5

doc/src/sgml/ecpg.sgml

 <!-- doc/src/sgml/ecpg.sgml -->
 
-<chapter id="ecpg">
+<chapter id="ecpg" xreflabel="ecpg">
  <title><application>ECPG</application> &mdash; Embedded <acronym>SQL</acronym> in C</title>
 
  <indexterm zone="ecpg"><primary>embedded SQL</primary><secondary>in C</secondary></indexterm>

doc/src/sgml/geqo.sgml

 <!-- doc/src/sgml/geqo.sgml -->
 
- <chapter id="geqo">
+ <chapter id="geqo" xreflabel="geqo">
   <title>Genetic Query Optimizer</title>
 
   <para>

doc/src/sgml/gin.sgml

 <!-- doc/src/sgml/gin.sgml -->
 
-<chapter id="gin">
+<chapter id="gin" xreflabel="gin">
 <title>GIN Indexes</title>
 
    <indexterm>

doc/src/sgml/gist.sgml

 <!-- doc/src/sgml/gist.sgml -->
 
-<chapter id="gist">
+<chapter id="gist" xreflabel="gist">
 <title>GiST Indexes</title>
 
    <indexterm>

doc/src/sgml/libpq.sgml

 <!-- doc/src/sgml/libpq.sgml -->
 
-<chapter id="libpq">
+<chapter id="libpq" xreflabel="libpq">
  <title><application>libpq</application> &mdash; C Library</title>
 
  <indexterm zone="libpq">

doc/src/sgml/plperl.sgml

 <!-- doc/src/sgml/plperl.sgml -->
 
- <chapter id="plperl">
+ <chapter id="plperl" xreflabel="PL/Perl">
   <title>PL/Perl &mdash; Perl Procedural Language</title>
 
   <indexterm zone="plperl">

doc/src/sgml/plpgsql.sgml

 <!-- doc/src/sgml/plpgsql.sgml -->
 
-<chapter id="plpgsql">
+<chapter id="plpgsql" xreflabel="PL/pgSQL">
   <title><application>PL/pgSQL</application> &mdash; <acronym>SQL</acronym> Procedural Language</title>
 
  <indexterm zone="plpgsql">

doc/src/sgml/plpython.sgml

 <!-- doc/src/sgml/plpython.sgml -->
 
-<chapter id="plpython">
+<chapter id="plpython" xreflabel="PL/Python">
  <title>PL/Python &mdash; Python Procedural Language</title>
 
  <indexterm zone="plpython"><primary>PL/Python</primary></indexterm>

doc/src/sgml/spi.sgml

 <!-- doc/src/sgml/spi.sgml -->
 
-<chapter id="spi">
+<chapter id="spi" xreflabel="spi">
  <title>Server Programming Interface</title>
 
  <indexterm zone="spi">

doc/src/sgml/vacuumlo.sgml

 <!-- doc/src/sgml/vacuumlo.sgml -->
 
-<refentry id="vacuumlo">
+<refentry id="vacuumlo" xreflabel="vacuumlo">
  <indexterm zone="vacuumlo">
   <primary>vacuumlo</primary>
  </indexterm>