include_if_exists facility for config file.

This works the same as include, except that an error is not thrown
if the file is missing. Instead the fact that it's missing is
logged.

Greg Smith, reviewed by Euler Taveira de Oliveira.

doc/src/sgml/config.sgml

@@ -89,6 +89,18 @@ include 'filename'
     Inclusions can be nested.
    </para>
 
+   <para>
+    <indexterm>
+     <primary><literal>include_if_exists</></primary>
+     <secondary>in configuration file</secondary>
+    </indexterm>
+    Use the same approach as the <literal>include</> directive, continuing
+    normally if the file does not exist.  A regular <literal>include</>
+    will stop with an error if the referenced file is missing, while
+    <literal>include_if_exists</> does not.  A warning about the missing
+    file will be logged.
+   </para>
+
    <para>
     <indexterm>
      <primary>SIGHUP</primary>