nspawn: lock down a few things in /proc by default

nspawn: lock down a few things in /proc by default

This tightens security on /proc: a couple of files exposed there are now
made inaccessible. These files might potentially leak kernel internals
or expose non-virtualized concepts, hence lock them down by default.
Moreover, a couple of dirs in /proc that expose stuff also exposed in
/sys are now marked read-only, similar to how we handle /sys.

The list is taken from what docker/runc based container managers
generally apply, but slightly extended.