lldpd: implement a reload hook

lldpd: implement a reload hook

Seems the default one is not working as expected.
The way that reload should work is that the 'start' service
call should return 1 (if lldpd is running) and then a normal
restart would be called.

However, for lldpd a reload would mean just clearing all custom TLVs
(if they're configured) and reloading the configuration.

So, this patch adds a reload hook, which would:
 - 'start' lldpd if it's not running (because we return 1 if not running)
 - reload configuration if it is running (also previously
    clearing custom TLVs if present)

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47367 3c298f89-4303-0410-b956-a3cf2f4a3e73