According to at least one report, there are some slightly insane servers out
there that have /dev/null set to non-user-writable.  This is totally broken,
but we want sshuttle to work with as many servers as possible, so let's fake
it up a bit instead.

We don't try to avoid /dev/null on the client; sshuttle needs root access
anyway, and if you're root, you can just fix your stupid /dev/null
permissions.