Fix neighbor discovery option parsing (#16)

Fix neighbor discovery option parsing (#16)

* make_option_mask: use `nd_` prefix for nd options

The prefix for neighbor discovery options was partially changed to
`nd6_` in d5700a48edf7006369f766d225e70bbca3d8055c. However
set_option_space continued to use `nd_`. This prevented either
`nd6_prefix_information` or `nd_prefix_information` from being parsed
since the former was in the wrong option space, and the latter did not
have the prefix removed.

* manual: fix nd_ and dhcp6_ option prefixes