Add types and accessors for working with half-aligned 64-bit values.

Add types and accessors for working with half-aligned 64-bit values.

Both OpenFlow and Netlink contain 64-bit fields that are only guaranteed
to be aligned on 32-bit boundaries.  This commit introduces types for
representing these fields and functions for working with them.  Followup
commits will make the OpenFlow and Netlink code use these types and
functions.