Improved handling of environment variables and parameters

As discussed in #55, the handling of environment variables is somewhat broken. So i've simplified the whole thing and through that also fixed the previously broken handling of the `SKYDNS_NAMESERVERS` environment variable as well as the broken `-machines` parameter.

@miekg ... don't forget to merge #55 as well ... and yes .. i am up to it :)
Just name it 'env'