state: fix shutdown when running in a container (FS#2425)

state: fix shutdown when running in a container (FS#2425)

Applies patch from @mikma [0] to fix Docker shutdown. Added detection to
state.c if running in a container or not.

Tested with a x86/64 Docker image.

Fixes FS#2425

CC: Mikael Magnusson <mikma@users.sourceforge.net>
CC: Petr Štetiar <ynezz@true.cz>
[0]: https://github.com/mikma/lxd-openwrt/blob/master/patches/procd-master/0003-docker-fix-problem-stopping-container.patch

Signed-off-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [coding style aligment]