Refactor, simplify registration of built-in watchdog daemon

This patch not only greatly simplifies the registration of the built-in
watchdog daemon, it also fixes a nasty race condition with an external
watchdog daemon.  The INIT_CMD_WDOG_HELLO could reach finit in reversed
order, leading to the built-in daemon shutting down, but finit killing
the just registered external daemon.

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

src/watchdog.h

@@ -27,9 +27,9 @@
 #define WDT_TIMEOUT 3
 
 #ifdef BUILTIN_WATCHDOG
-void    watchdog(char *progname);
+int     watchdog(char *progname);
 #else
-#define watchdog(progname)
+#define watchdog(progname) 0
 #endif /* BUILTIN_WATCHDOG */
 
 /**