Relocate ismnt() and fismnt() to util.c, for sharing with initctl

initctl cannot link with helpers.c, so let's relocate these helper
functions to util.c instead.  Need them to probe for cgroup support.

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

plugins/modprobe.c

@@ -48,7 +48,7 @@
 
 #include "config.h"
 #include "finit.h"
-#include "helpers.h"
+#include "util.h"
 #include "plugin.h"
 
 struct module {