These are headers (and sources) contributed directly from FreeBSD which are
required to build bhyve.  Updates are made to our copies here when corresponding
updates to bhyve require them.

With few exceptions (noted below) they are verbatim copies taken from the
FreeBSD source tree.  Files requiring extensive modification are likely a
candidate to exist as a shim in usr/src/compat/bhyve.  Conversely, if only a
tiny modification is needed to make a file suitable for contrib, that trade-off
is probably worthwhile (if done with a '#ifdef _FreeBSD_' guard for clarity).

Files modified for compatibility:
- isa/rtc.h