test cross os builds for plan9, openbsd and darwin

thanks qbit :)
Split FileInfo into separate files that can be used per OS.

This allows neinp to be used on Darwin and plan9.
add go vet and golint to build
only mail on build failure
use custom go.rbn.im import path
fix url in path, now point to sr.ht url
fix git url in .build.yml
use archlinux in .build.yml
move to sr.ht and go modulize