jail: include necessary files for per-netns netifd instance

jail: include necessary files for per-netns netifd instance

Up to now only proto 'static' and 'none' have been supported for
in-jail interfaces as Shell-based protocol handlers needed some more
utilities, PATH set in the environment and /var/ubus/ubus.sock to be
mounted on the per-netns-jail ubus socket.
Include files needed by shell proto handler, set PATH in env and bend
ubus socket path to get DHCP client to work for per-jail netns.

Signed-off-by: Daniel Golle <[email protected]>