WIP: experimental vendor cleanup

This is a funny situation.

I did a little test

rm -rf vendor/
git checkout vendor/vendor.json
govendor sync

I would have expected no changes but that's not true.

Most of the changes are just AUTHORS and LICENSE that may just be realted to some pruning feature of govendor (if present), but there are some removal of go files in golang.org/x/sys/unix package that left me perplexed.

/cc: @tmaczukin @ayufan