initctl: refactor pid_cgroup(), return allocated buffer

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

src/cgutil.h

@@ -56,7 +56,7 @@ int cgroup_avail(void);
 
 char *pid_cmdline (int pid);
 char *pid_comm    (int pid, char *buf, size_t len);
-char *pid_cgroup  (int pid, char *buf, size_t len);
+char *pid_cgroup  (int pid);
 
 uint64_t cgroup_memory(char *group);