All Debian based distros are moving towards a /etc/network/interfaces.d/
world. Adding support for that is too complex, so we're reverting back
to calling 'ifup -a' or 'ifdown -a'. We still guard this by checking
both for 'ifup' and the existence of /etc/network/interfaces (which in
the new world only contains a line to source per-interface files).
By popular demand, this patch also changes to bring up/down networking
in the background. Useful for embedded system bringup and containers.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>