- Add a manual page entity for m4(1)

- Add filename tags with a little rewording.

I'm not happy with the way the sendmail.cf generation is covered (the
Handbook is still using the old way, different from the use of the
/etc/Makefile), I will fix it after the doc/ freeze.

en_US.ISO8859-1/books/handbook/mail/chapter.sgml

@@ -1163,7 +1163,7 @@ freefall		MX	20	who.cdrom.com</programlisting>
 
     <para>This extracts to only a few hundred kilobytes.  The file
       <filename>README</filename> in the <filename>cf</filename>
-      directory can serve as a basic introduction to m4
+      directory can serve as a basic introduction to &man.m4.1;
       configuration.</para>
 
     <para>The best way to support UUCP delivery is to use the
@@ -1254,9 +1254,9 @@ if-bus.UUCP                   uucp-dom:if-bus
 
     <para>As a reminder that this file needs to be converted into a
       DBM database file before use.  The command line to accomplish
-      this is best placed as a comment at the top of the mailertable.
+      this is best placed as a comment at the top of the <filename>mailertable</filename> file.
       You always have to execute this command each time you change
-      your mailertable.</para>
+      your <filename>mailertable</filename> file.</para>
 
     <para>Final hint: if you are uncertain whether some particular
       mail routing would work, remember the <option>-bt</option>