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
Merged-In: Id0b4dc18dd219b4d35846e161022a37c8de3e3bb
Change-Id: Id0b4dc18dd219b4d35846e161022a37c8de3e3bb
2 files changed