Yield execution in forked children back to PID 1

- Speeds up start-up a bit
 - Ensures the child is moved to its cgroup as soon as possible

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/helpers.h

@@ -26,6 +26,7 @@
 #define FINIT_HELPERS_H_
 
 #include <mntent.h>
+#include <sched.h>		/* sched_yield() */
 #include <stdarg.h>
 #include <sys/resource.h>
 #include <sys/types.h>