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>

Makefile.am

@@ -19,6 +19,9 @@ pkginclude_HEADERS = cond.h finit.h helpers.h inetd.h plugin.h svc.h
 if INETD
 finit_SOURCES     += inetd.c	inetd.h
 endif
+if WATCHDOGD
+finit_SOURCES     += watchdog.c	watchdog.h
+endif
 
 finit_CFLAGS       = -W -Wall -Wextra -std=gnu99 $(lite_CFLAGS) $(uev_CFLAGS)
 finit_LDADD        = $(lite_LIBS) $(uev_LIBS)