rpcd-mod-rrdns: add new rpcd plugin for rapid reverse DNS lookups

The plugin provides a new ubus procedure "network.rrdns.lookup" which
allows looking up the names of a large chunk of IP addresses at once,
within a fixed global timeout.

This is useful to have a guaranteed maximum query time even if the
local DNS setup is broken or timing out.

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