jail: refactor default mounts into new structure

jail: refactor default mounts into new structure

Add default mounts of /dev, /dev/pts, /dev/shm, /proc and /sys to
the restructured mounts AVL list instead of calling mount directly.
While for slim containers this change shouldn't make any difference,
it allows OCI containers to override options of those default
filesystems.
The previous hack keeping /proc/sys/net mounted read-write if inside
a new network namespace while all the rest of /proc/sys is read-only
cannot easily be translated and is removed for now.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>