procd: update to the latest version, add "button" hotplug handler

procd: update to the latest version, add "button" hotplug handler

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
In 15.05-rc2 we got support for "set-interval" and "clear-interval"
which got dropped with the update of procd done in r46257. This new
"button" action provides similar functionality so it makes sense to
backport it to 15.05.

Backport of r46578

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46579 3c298f89-4303-0410-b956-a3cf2f4a3e73