libxtables: move afinfo around

libxtables should not rely on the program executable providing the
magic constants for using [gs]etsockopt.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/iptables-save.c b/iptables-save.c
index d08ec4b..c4306fd 100644
--- a/iptables-save.c
+++ b/iptables-save.c
@@ -141,6 +141,7 @@
 
 	xtables_program_name = program_name;
 	xtables_init();
+	xtables_set_nfproto(NFPROTO_IPV4);
 #ifdef NO_SHARED_LIBS
 	init_extensions();
 #endif