Support parsing ND option messages.

Define a new StructNduseroptmsg class that is a rough equivalent
of the "struct nduseroptmsg" used to pass RA options from the
kernel to userspace. Also define a new NdOption class and make
the existing pref64 option subclass it.

Bug: 153694684
Test: new unit tests
Change-Id: I3b71e63ee2cdaa40d095e889188943c5b0cd13af
Merged-In: I3b71e63ee2cdaa40d095e889188943c5b0cd13af
(cherry picked from commit 48d23146)