basic: move container_get_leader() to process-util.[ch]

basic/util.[ch] is a grab-bag of unrelated functions. Let's move a few
of the remaning functions to better locations.

src/basic/process-util.h

@@ -50,6 +50,8 @@ int get_process_environ(pid_t pid, char **ret);
 int get_process_ppid(pid_t pid, pid_t *ret);
 int get_process_umask(pid_t pid, mode_t *ret);
 
+int container_get_leader(const char *machine, pid_t *pid);
+
 int wait_for_terminate(pid_t pid, siginfo_t *status);
 
 typedef enum WaitFlags {