Doc: add XML ID attributes to <sectN> and <varlistentry> tags.

This doesn't have any external effect at the moment, but it
will allow adding useful link-discoverability features later.

Brar Piening, reviewed by Karl Pinc.

Discussion: https://postgr.es/m/CAB8KJ=jpuQU9QJe4+RgWENrK5g9jhoysMw2nvTN_esoOU0=a_w@mail.gmail.com

doc/src/sgml/sepgsql.sgml

@@ -315,7 +315,7 @@ $ sudo semodule -r sepgsql-regtest
 
  <sect2 id="sepgsql-features">
   <title>Features</title>
-  <sect3>
+  <sect3 id="sepgsql-features-controlled-obj-classes">
    <title>Controlled Object Classes</title>
    <para>
     The security model of <productname>SELinux</productname> describes all the access
@@ -348,7 +348,7 @@ $ sudo semodule -r sepgsql-regtest
    </para>
   </sect3>
 
-  <sect3>
+  <sect3 id="sepgsql-features-dml-permissions">
    <title>DML Permissions</title>
 
    <para>
@@ -432,7 +432,7 @@ UPDATE t1 SET x = 2, y = func1(y) WHERE z = 100;
    </para>
   </sect3>
 
-  <sect3>
+  <sect3 id="sepgsql-features-ddl-permissions">
    <title>DDL Permissions</title>
    <para>
     <productname>SELinux</productname> defines several permissions to control common
@@ -523,7 +523,7 @@ UPDATE t1 SET x = 2, y = func1(y) WHERE z = 100;
 
   </sect3>
 
-  <sect3>
+  <sect3 id="sepgsql-features-trusted-procedures">
    <title>Trusted Procedures</title>
    <para>
     Trusted procedures are similar to security definer functions or setuid
@@ -579,7 +579,7 @@ postgres=# SELECT cid, cname, show_credit(cid) FROM customer;
    </para>
   </sect3>
 
-  <sect3>
+  <sect3 id="sepgsql-features-dynamic-domain-transitions">
    <title>Dynamic Domain Transitions</title>
    <para>
     It is possible to use SELinux's dynamic domain transition feature
@@ -641,7 +641,7 @@ ERROR:  SELinux: security policy violation
    </para>
   </sect3>
 
-  <sect3>
+  <sect3 id="sepgsql-features-misc">
    <title>Miscellaneous</title>
    <para>
     We reject the <link linkend="sql-load"><command>LOAD</command></link> command across the board, because