luci-base: dispatcher.lua: support "absent" fs dependency for menu nodes
luci-base: dispatcher.lua: support "absent" fs dependency for menu nodes

The "absent" dependency type requires the given path to not exist on the
local system for the condition to be satisified. This is useful to disable
menu nodes depending on the presence of specific files.

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

hnyman/luci,openwrt/luci,hnyman/luci,hnyman/luci,hnyman/luci,openwrt/luci,openwrt/luci,hnyman/luci,openwrt/luci,openwrt/luci,hnyman/luci,openwrt/luci,openwrt/luci,hnyman/luci,hnyman/luci,openwrt/luci