Reduce fork calls in daemon

The forked child process is a grandchild process from the viewpoint of
the process which invoked the caller process.  That means the child is
detached at that point, and it does not need to fork twice.

process.c

@@ -7109,20 +7109,14 @@ rb_daemon(int nochdir, int noclose)
 #else
     int n;
 
-#define fork_daemon() \
-    switch (rb_fork_ruby(NULL)) { \
-      case -1: return -1; \
-      case 0:  break; \
-      default: _exit(EXIT_SUCCESS); \
+    switch (rb_fork_ruby(NULL)) {
+      case -1: return -1;
+      case 0:  break;
+      default: _exit(EXIT_SUCCESS);
     }
 
-    fork_daemon();
-
     if (setsid() < 0) return -1;
 
-    /* must not be process-leader */
-    fork_daemon();
-
     if (!nochdir)
         err = chdir("/");