Add support for registering a watchdog daemon

This patch adds initial support for registering a watchdog daemon with
Finit.  Needed for situations when you want the system to actually be
rebooted by the watchdog timer (WDT) circuitry rather than the kernel.

The initctl API has been extended with a INIT_CMD_WDOG_HELLO command
that is used to both register and "ping" Finit periodically.  As long as
Finit responds (with an ACK) the watchdog daemon knows Finit is alive.

At shutdown Finit stops all services (except kernel threads and daemons
marked with @ in the first character of their process name) and, if a
watchdog has registered, SIGPWR is sent to it when Finit has finished
unmounting etc., just prior to kernel reboot/halt/poweroff.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

finit.h

@@ -67,6 +67,7 @@
 #define INIT_CMD_QUERY_INETD    14
 #define INIT_CMD_EMIT           15
 #define INIT_CMD_GET_RUNLEVEL   16
+#define INIT_CMD_WDOG_HELLO     128  /* Watchdog register and hello */
 #define INIT_CMD_NACK           254
 #define INIT_CMD_ACK            255
 
@@ -81,6 +82,7 @@ struct init_request {
 extern int    debug;
 extern int    quiet;
 extern int    silent;
+extern int    wdogpid;
 extern int    runlevel;
 extern int    cfglevel;
 extern int    prevlevel;