Fix parsing a NduseroptMessage inside a NetlinkMessage.

The parsing code inside NduseroptMessage assumed that the message
started at the beginning of the buffer. Fix this, and also
restore the original endianness and limit of the buffer after
parsing.

Add tests for this, and make the existing tests a bit more
readable.

Bug: 153694684
Test: new unit tests
Change-Id: Id0b4dc18dd219b4d35846e161022a37c8de3e3bb