Update a few build tags to be more generic, ...

... add a couple more SETNS constants, and update Travis with a bunch of fixes/tweaks (including removing the nonfunctional cross-compile stuff for now)

I'm working on discussing with the Travis folks what would be necessary to get our cgo + Docker-based workers + cross compile (yes, I want to bake my own cake, have it for myself, and eat it all alone), but for now, I've commented out "386" and "arm" (which will be the only compilation targets we support).