process-util: add simple wrapper around PR_SET_CHILD_SUBREAPER

Let's a simple helper that knows how to deal with PID == 1.

src/basic/process-util.h

@@ -209,3 +209,4 @@ _noreturn_ void freeze(void);
 int get_process_threads(pid_t pid);
 
 int is_reaper_process(void);
+int make_reaper_process(bool b);