Fix install-html-split target:

With the current code running "make install" will build the files but
will install only 2 files (docbook.css and tramemark.html).  Another
"make install" is required to get the missing files.  This is not the
expected behavior.  A quick workaround is to run "make && make
install", but let's fix the code to get a working "make install".

Discussed with:	gabor

share/mk/doc.docbook.mk

@@ -705,11 +705,7 @@ install-${_curformat}: ${DOC}.${_curformat}
 .endif
 	@[ -d ${DESTDIR} ] || ${MKDIR} -p ${DESTDIR}
 .if ${_cf} == "html-split"
-.for f in ${_html_docs}
-.if exists(${f})
-	${INSTALL_DOCS} ${f} ${DESTDIR}
-.endif
-.endfor
+	${INSTALL_DOCS} $$(${XARGS} < HTML.manifest) ${DESTDIR}
 .else
 	${INSTALL_DOCS} ${.ALLSRC} ${DESTDIR}
 .endif