libiptc: combine common types

Make an xt_chainlabel type out of ipt_chainlabel and ip6t_chainlabel,
and add backward-API #defines. The ABI naturally does not change
either, so no soversion bump.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/include/Makefile.am b/include/Makefile.am
index 0a1abea..6f7da59 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -9,4 +9,4 @@
 
 nobase_include_HEADERS += \
 	libiptc/ipt_kernel_headers.h libiptc/libiptc.h \
-	libiptc/libip6tc.h libiptc/libxtc.h
+	libiptc/libip6tc.h libiptc/libxtc.h libiptc/xtcshared.h