Update PACKAGERS.md and hack/make/ubuntu

While I was in PACKAGERS.md updating our dependency list to no longer include LXC, I noticed a lot of things that could be improved, and ended up rewriting most of the language in the document (especially since we've received some negative feedback on this doc previously).  I've added to it a bunch of our newer build system features, like `AUTO_GOPATH` (that quite a few of our packagers were very happy to see and use).

Also, while I was there, I noticed that we no longer require "ip" (from iproute2) either, so our official "runtime" deps are down to just "iptables" for sure, with "git" and "xz" being necessary depending on how you use Docker.  That's pretty impressive, IMHO.

Anyone looking to help proofread or just curious about how the new doc looks, the easiest way to check it out is via https://github.com/tianon/docker/blob/update-packagers-doc-and-lxc-dep/hack/PACKAGERS.md