procd: switch buttons to the new "button" hotplug handler

This allows requesting "timeout" ACTION in script by returning a
positive value.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46580 3c298f89-4303-0410-b956-a3cf2f4a3e73