Replace dns.mod

[thommey]

#333 and other common issues with dns.mod lead to the usual advice "disable dns.mod and try again", and especially facing #333, there might be other 3rd party resolvers we can include

[michaelortmann]

A libunbound based asynchronous dns resolve module for eggdrop:

https://gitlab.com/mortmann/unbound.mod

IRC discussion resulted in developing this mod separate from eggdrop for now, but also analyze a merge with dns.mod.