Update PACKAGERS.md with some text changes, including and especially moving "LXC" to a new "Optional Dependencies" section

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Reflow PACKAGERS.md to 80 columns (where possible) and update some minor formatting

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Mention in PACKAGERS.md that Tianon is available for packager needs

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Rewrite more loads of PACKAGERS.md to hopefully remove some outdated information, add some updated information and pointers, and generally make the tone of this document less condescending :)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update package deps to remove LXC and move aufs-tools to be in "Recommends" instead of strict Depends

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Update the relevant docs section to match PACKAGERS.md

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add "git" to our Ubuntu package "Recommends" (whoops, this should've been in here for a while now)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Add further clarification of where/when the build-time deps are applicable

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4463 from tianon/update-packagers-doc-and-lxc-dep

Update PACKAGERS.md and hack/make/ubuntu