doc: add VERSION section to manpages

This shall make it easier to identify outdated HTML renditions on the
interwebs, since many of them do not display the .TH header like man(1)
does.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/ip6tables.8.in b/ip6tables.8.in
index 5688133..4306934 100644
--- a/ip6tables.8.in
+++ b/ip6tables.8.in
@@ -419,3 +419,6 @@
 .\" .. sexy, too ..
 .\" .. witty, charming, powerful ..
 .\" .. and most of all, modest ..
+.SH VERSION
+.PP
+This manual page applies to ip6tables @PACKAGE_VERSION@.