Ship documentation without intermediate tarballs

Documentation files in HTML and man formats are now prepared for
distribution using the distprep make target, like everything else.  They
are placed in doc/src/sgml/html and manX and installed from there by
make install, if present.  The business with the tarballs in the tarball
is gone.

doc/src/sgml/stylesheet.xsl

@@ -9,6 +9,7 @@
 
 
 <!-- Parameters -->
+<xsl:param name="base.dir" select="'html'"></xsl:param>
 <xsl:param name="html.stylesheet" select="'stylesheet.css'"></xsl:param>
 <xsl:param name="use.id.as.filename" select="'1'"></xsl:param>
 <xsl:param name="make.valid.html" select="1"></xsl:param>