add daemon.h and documentation on functions

ccan/daemon-with-notify/daemon.h

+#ifndef CCAN_DAEMON_WITH_NOTIFY_H
+#define CCAN_DAEMON_WITH_NOTIFY_H
+
+/**
+ * daemonize - turns this process into a daemon
+ *
+ * This routine will fork() us off to become a daemon. It will return
+ * -1 on error and 0 on success.
+ *
+ * It has a few optional behaviours:
+ * @nochdir: if nochdir is set, it won't chdir to /
+ *   this means we could hold onto mounts
+ * @noclose: if noclose is set, we won't close stdout, stdin and stderr.
+ * @wait_sigusr1: if wait_sigusr1 is set, the parent will not exit until the
+ *   child has either exited OR it receives a SIGUSR1 signal. You can use this
+ *   to have the parent only exit when your process has done all the
+ *   danegerous initialization that could cause it to fail to start
+ *   (e.g. allocating large amounts of memory, replaying REDO logs).
+ *   This allows init scripts starting the daemon to easily report
+ *   success/failure.
+ */
+int daemonize(int nochdir, int noclose, int wait_sigusr1);
+
+/**
+ * daemon_is_ready - signals parent that it can exit, we started okay
+ *
+ * After a daemonize() call, this function will send a SIGUSR1 to the parent
+ * telling it to exit as we have started up okay.
+ */
+int daemon_is_ready(void);
+
+#endif /* CCAN_DAEMON_WITH_NOTIFY_H */