javadoc: allow user-specified footers on all generated pages

For improved end-user customization and to promote style consistency, it would be helpful if javadoc had a feature to insert user-specified text that would appear at the bottom of every javadoc-generated output page.

For example, Sun's standard copyright notice could appear as:

    <hr>
    <font size=-2><a href="../relnotes/SMICopyright.html">Copyright &#169;</a>
    1997 Sun Microsystems, Inc. 2550 Garcia Ave., Mtn. View, CA 94043-1100 USA.
    All rights reserved.</font><br>
   <font size=-1><em>February 26, 1997</em></font>