Add fallback for betoh/htobe if they do not exist

For older systems endian.h may not contain the betoh/htobe macros.
If they are missing, add them and make sure that any user of
htobe/betoh uses the compat/endian.h

Signed-off-by: Jesper Derehag <jderehag@hotmail.com>
Tested-by: Jesper Derehag <jderehag@hotmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>