radvd: eliminate confusing warnings when stop service

When run 'service radvd stop' in case radvd is not started/running,
we'll get warnings like this:

Stopping radvd: no /usr/sbin/radvd found; none killed
/etc/init.d/radvd: warning: cannot restore settings
radvd.

We could by pass these OPs by just check if the service is running
or not so eliminate possible warnings.

Signed-off-by: Liang Li <liang.li@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>