init: Implement 'exec' command.

(cherry-pick of d05ab3952ec0e38f33a0e80ce6b9eb45b0064ba4.)

Change-Id: Id6d9bb32e51a0ad090ed8240cc505dc45b57b35d
diff --git a/init/init.c b/init/init.c
index d3dd019..096d898 100644
--- a/init/init.c
+++ b/init/init.c
@@ -126,7 +126,7 @@
     return -1;
 }
 
-static void zap_stdio(void)
+void zap_stdio(void)
 {
     int fd;
     fd = open("/dev/null", O_RDWR);