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>

src/helpers.h

@@ -130,9 +130,6 @@ static inline int dprint(int fd, const char *s, size_t len)
 	return rc;
 }
 
-int	ismnt		(char *file, char *dir, char *mode);
-int	fismnt		(char *dir);
-
 #endif /* FINIT_HELPERS_H_ */
 
 /**