Fix add_ra_option_lowpanco() and add_ra_option_abro() on both big-endian and little-endian platforms.

[trofi]

It looks like code did not work before as code serialized 16-bit values in host-endian format.

Three fixes here:

• use htons to serialize uint16_t in endianness-agnostic way
• use uint8_t for bitfield to serialize it in compiler-agnostic way
• fix test expectations

tested on x86_64 and powerpc32