bb_daemonize_or_rexec(): add flag to double-fork; use it in start-stop-daemon

Add a DAEMON_DOUBLE_FORK flag to make bb_daemonize double-fork so it isn't a
session leader, and hence doesn't get a controlling tty on Linux if a tty is
ever opened, similar to how libdaemon's daemon_fork or the big
start-stop-daemon does it - And use it in start-stop-daemon.

For details, see http://www.win.tue.nl/~aeb/linux/lk/lk-10.html#ss10.3

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -966,6 +966,7 @@ enum {
 	DAEMON_DEVNULL_STDIO = 2,
 	DAEMON_CLOSE_EXTRA_FDS = 4,
 	DAEMON_ONLY_SANITIZE = 8, /* internal use */
+	DAEMON_DOUBLE_FORK = 16, /* double fork to avoid controlling tty */
 };
 #if BB_MMU
   enum { re_execed = 0 };