Adapt ifupdown support to new world, run 'ifup -a' in the background
Adapt ifupdown support to new world, run 'ifup -a' in the background

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 <583c295fd7602c168ad814279bbc3894ba65f5d6@gmail.com>

troglobit/finit,troglobit/finit