luci-mod-network: fix handling of optional RA/NDP options

Fix the handling of optional IPv6 RA and NDP options
that were exposed to LuCI with 504bdb23

Commit 504bdb23

 defined them optional but provided default values.
Those values might get unnecessarily written to /etc/config dhcp when
the the user modifies some other values. Remove the default values,
but provide placeholder for some of them.

Add the missing optional definition to 'ndproxy_routing'.
(It is a flag, so optional default values do not get written to
the config file.)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>