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.c

@@ -50,6 +50,7 @@
 int   debug     = 0;
 int   quiet     = QUIET_MODE;	/* Delayed disable of silent mode. */
 int   silent    = SILENT_MODE;	/* Completely silent, including boot */
+int   wdogpid   = 0;		/* No watchdog by default */
 int   runlevel  = 0;		/* Bootstrap 'S' */
 int   cfglevel  = RUNLEVEL;	/* Fallback if no configured runlevel */
 int   prevlevel = -1;