watchdog: add support for starting/stopping kernel watchdog

watchdog: add support for starting/stopping kernel watchdog

Extend the ubus watchdog cmd with the parameter magicclose; when set and
in case the stopped parameter is enabled the kernel watchdog will be stopped
by first sending the magic character 'V' followed by a close of the watchdog
fd. In case stopped is set to disabled the watchdog fd will be created again.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Acked-by: John Crispin <john@phrozen.org>