init: Implement 'exec' command.

(cherry-pick of d05ab3952ec0e38f33a0e80ce6b9eb45b0064ba4.)

Change-Id: Id6d9bb32e51a0ad090ed8240cc505dc45b57b35d
diff --git a/init/init.h b/init/init.h
index e03bd53..654a80b 100644
--- a/init/init.h
+++ b/init/init.h
@@ -149,5 +149,6 @@
 extern struct selabel_handle *sehandle;
 extern struct selabel_handle *sehandle_prop;
 extern int selinux_reload_policy(void);
+void zap_stdio(void);
 
 #endif	/* _INIT_INIT_H */