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>

watchdog.h

@@ -0,0 +1,40 @@
+/* Built-in watchdog daemon
+ *
+ * Copyright (c) 2016  Joachim Nilsson <troglobit@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "config.h"		/* Generated by configure script */
+
+#define WDT_DEVNODE "/dev/watchdog"
+#define WDT_TIMEOUT 3
+
+#ifdef BUILTIN_WATCHDOG
+void    watchdog(char *progname);
+#else
+#define watchdog(progname)
+#endif /* BUILTIN_WATCHDOG */
+
+/**
+ * Local Variables:
+ *  indent-tabs-mode: t
+ *  c-file-style: "linux"
+ * End:
+ */