core,shutdown: don't bother with unmounting any mounts below /sys, /proc, /dev when...

core,shutdown: don't bother with unmounting any mounts below /sys, /proc, /dev when shutting down

After all, mounts below these directories are pretty much guaranteed to
be virtual, and it's hence unnecessary to unmount them during shutdown.
Moreover, in less-priviliged containers we might lack the rights to
unmount them, hence don't even try.

http://lists.freedesktop.org/archives/systemd-devel/2015-January/027113.html