watchdog: Add simple built-in watchdog daemon

This patch adds a simple but effective watchdog daemon with support for
handover to an external daemon, should one register using the API.

Disabled by default, enable by:

    configure --enable-watchdog

The watchdog forks to the background, opening /dev/watchdog setting a
three second timeout, and kicking every second.  At shutdown/reboot the
daemon will receive SIGPWR from Finit to exit and cause a WDT reboot.

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

finit.c

@@ -46,6 +46,7 @@
 #include "sm.h"
 #include "tty.h"
 #include "utmp-api.h"
+#include "watchdog.h"
 
 int   debug     = 0;
 int   quiet     = QUIET_MODE;	/* Delayed disable of silent mode. */
@@ -343,6 +344,11 @@ int main(int argc, char* argv[])
 	 */
 	conf_parse_config();
 
+	/*
+	 * Start built-in watchdog as soon as possible, if enabled
+	 */
+	watchdog(argv[0]);
+
 	/* Set hostname as soon as possible, for syslog et al. */
 	set_hostname(&hostname);