doc: Distribute a modern version of texinfo.tex with the docs

* doc/texinfo.tex: Ship a more recent texinfo.tex, from texinfo 6.1, so we
  don't have to worry about that.  E.g., this should allow UTF-8 unicode
  chars to be used directly in the text.
* doc/Makefile.am: Add previous to EXTRA_DIST
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 943f06b..00d404a 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -112,7 +112,8 @@
 	ripngd.8 pimd.8 vtysh.1 watchquagga.8 zebra.8 \
 	mpls/ChangeLog.opaque.txt mpls/cli_summary.txt \
 	mpls/opaque_lsa.txt mpls/ospfd.conf \
-	$(figures_sources) $(figures_png) $(figures_txt)
+	$(figures_sources) $(figures_png) $(figures_txt) \
+	texinfo.tex
 
 draft-zebra-00.txt: draft-zebra-00.ms
 	groff -T ascii -ms $< > $@