Reimplement daemonize() without using daemon()

The daemon() helper function is widely implemented, but is not really
standardized and is actually somewhat buggy on some platforms.

Implementing the desired functionality is also not all that complicated.

So, do it ourselves.

Closes: gh-161

nbd-server.c

@@ -3779,18 +3779,24 @@ void setup_servers(GArray *const servers, const gchar *const modernaddr,
 
 /**
  * Go daemon (unless we specified at compile time that we didn't want this)
- * @param serve the first server of our configuration. If its port is zero,
- * 	then do not daemonize, because we're doing inetd then. This parameter
- * 	is only used to create a PID file of the form
- * 	/var/run/nbd-server.<port>.pid; it's not modified in any way.
  **/
 #if !defined(NODAEMON)
 void daemonize() {
 	FILE*pidf;
 
-	if(daemon(0,0)<0) {
-		err("daemon");
-	}
+        pid_t child=fork();
+        if(child < 0) {
+		err("fork");
+	} else if(child > 0) {
+                exit(EXIT_SUCCESS);
+	} else {
+                if(setsid() < 0) {
+                        err("setsid");
+                }
+        }
+        if(chdir("/")<0) {
+                err("chdir");
+        }
 	if(!*pidfname) {
 		strncpy(pidfname, "/var/run/nbd-server.pid", 255);
 	}
@@ -3802,9 +3808,28 @@ void daemonize() {
 		perror("fopen");
 		fprintf(stderr, "Not fatal; continuing");
 	}
+        int newfd;
+        if((newfd = open("/dev/null", O_RDWR)) < 0) {
+                err("open");
+        }
+        if(dup2(0, newfd) < 0) {
+                err("dup2 stdin");
+        }
+        if(dup2(1, newfd) < 0) {
+                err("dup2 stdout");
+        }
+        if(dup2(2, newfd) < 0) {
+                err("dup2 stderr");
+        }
+        child=fork();
+        if(child < 0) {
+                err("fork");
+        } else if(child > 0) {
+                exit(EXIT_SUCCESS);
+        }
 }
 #else
-#define daemonize(serve)
+#define daemonize()
 #endif /* !defined(NODAEMON) */
 
 /*