These are the compatibility headers for building bhyve on illumos.  They are
comprised of the shims and glue needed to use native functionality as the
backing for FreeBSD interfaces.  It often means heavy use of #include_next,
#define renames, and forward definitions to some glue functions.

For headers which would otherwise be copied verbatim from FreeBSD, the
usr/src/contrib/bhyve area is the appropriate home.  This allows us to carry
only the bare minimum in the 'compat' headers while making updates for the
'contrib' headers easy (simply copy over the new version).