Check if tun device is avaliable for installation

[zelahi]

Address Issue #101

It checks to see if /dev/net/tun exists for the ce installer. However it does allow the user to install and set the binaries if they wish to bypass this check

[GordonTheTurtle]

Please sign your commits following these rules: https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work The easiest way to do this is to amend the last commit:

$ git clone -b "add-check-for-tun-device" [email protected]:zelahi/docker-install.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[thaJeztah]

Rebased to trigger CI

@AkihiroSuda PTAL

[AkihiroSuda]

Can we print hint like “modprobe tun”?