Add daemonize function to fork off as a daemon.

Method:
1. Forks off
2. Becomes session leader
3. Forks off again
4. Sets up new environment
5. Closes all open file descriptors

Squashed commit of the following:

commit 1e71e5dcc6f979ce2f06d59e3c05660fa97e69f3
Author: Tushar Pankaj <tushar.s.pankaj@gmail.com>
Date:   Thu Nov 1 16:28:09 2018 -0500

    Fix missing include in daemonize

    Signed-off-by: Tushar Pankaj <tushar.s.pankaj@gmail.com>

commit 80aaa1112f2bf21f98be02f9e5f8be4bc7ed94d1
Author: Tushar Pankaj <tushar.s.pankaj@gmail.com>
Date:   Thu Nov 1 16:23:34 2018 -0500

    First draft of daemonize

    Signed-off-by: Tushar Pankaj <tushar.s.pankaj@gmail.com>

2 files changed, 68 insertions, 0 deletions

diff --git a/daemonize.c b/daemonize.c
new file mode 100644
index 0000000..4f480e5
--- /dev/null
+++ b/daemonize.c
@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Copyright (C) 2018 Wireguard LLC
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+void daemonize(void) {
+    pid_t pid;
+
+    /* fork */
+    pid = fork();
+
+    /* check fork for error */
+    if (pid < 0) {
+        perror("fork error");
+        exit(EXIT_FAILURE);
+    }
+
+    /* terminate the parent */
+    if (pid > 0) {
+        exit(EXIT_SUCCESS);
+    }
+
+    /* become session leader */
+    if (setsid() < 0) {
+        perror("setsid error");
+        exit(EXIT_FAILURE);
+    }
+
+    /* fork */
+    pid = fork();
+
+    /* check fork for error */
+    if (pid < 0) {
+        perror("fork error");
+        exit(EXIT_FAILURE);
+    }
+
+    /* terminate the parent */
+    if (pid > 0) {
+        exit(EXIT_SUCCESS);
+    }
+
+    /* set up new environment */
+    umask(S_IRWXG | S_IRWXO); /* umask 077 */
+    chdir("/"); /* cd / to avoid locking up original cwd */
+
+    /* close file descriptors */
+    for (int fd = sysconf(_SC_OPEN_MAX); fd >=0; fd--) {
+        close(fd);
+    }
+}
diff --git a/daemonize.h b/daemonize.h
new file mode 100644
index 0000000..3720ff1
--- /dev/null
+++ b/daemonize.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Copyright (C) 2018 Wireguard LLC
+ */
+
+#ifndef DAEMONIZE_H
+#define DAEMONIZE_H
+
+void daemonize(void);
+
+#endif