Fix for interaction issue between page numbers and html literal blocks

As tfkowal reported ( http://www.pgdp.net/phpBB2/viewtopic.php?p=1000196#1000196 )
if a .pn command occurs and there is no eligible insertion spot before a .li
then we get HTML that will not validate.

ppgen issues a warning, but then plops the page number info onto the start
of the first line inside the .li block.

The problem tom Tom spotted is that he ended up with a line
reading: &lt;span...> page number info &lt;/span>&lt;table> table info...
and then the ppgen code to detect "naked" page numbers and enclose them in
a div did not work. So he ended up with a naked span, and code
that would not validate.

This fix just leaves the page number just before the .li. It will later
become part of a paragraph comprising only the page number, but at least
the code will validate, and the user has the warning message to alert him
that something may be amiss.
Enhanced warning for .li encountered when placing page number

The message now includes the page number, to help the PPer figure out where
the problem occurred.
Merge pull request #61 from wf49670/develop

Fix for interaction issue between page numbers and html literal blocks. Credit: Walt. Thanks.