/e/n/i: move sourcing of /etc/network/interfaces.d/ to begin of file

netcardconfig doesn't properly handle this section and it disappears
when configuring network devices then.

Rewriting netcardconfig is on our agenda (see
https://github.com/grml/grml-network/issues/11), until we get there
let's make sure the status quo works as good as possible.

Take over wording based on what ifupdown's postinst script uses.