Having fou.go build only for linux breaks builds for darwin:
```
$ go build main.go
src/github.com/vishvananda/netlink/fou_unspecified.go:5:15: undefined: Fou
src/github.com/vishvananda/netlink/fou_unspecified.go:9:15: undefined: Fou
src/github.com/vishvananda/netlink/fou_unspecified.go:13:26: undefined: Fou
```
Instead, build fou.go for all platforms since it doesn't have platform-specific code:
```
$ go build main.go
$ ./main
not implemented
```
chg: addtl comment and made minor logic optimization as disscussed in PR #296
chg: flipped Persist to NonPersist
chg: comments, only unpersist tuntap if flag is set
chg: tuntap persist optional, allow empty intfc name
chg: added conditional build
Signed-off-by: Ralph Schmieder <ralph.schmieder@gmail.com>
Sean Karlage
Ralph Schmieder
Alessandro Boch
Reinier Schoof