LU-2675 libcfs: add libcfs/byteorder.h

LU-2675 libcfs: add libcfs/byteorder.h

Move the byte swapping macros out of libcfs/posix/libcfs.h and into a
new header libcfs/byteorder.h which includes asm/byteorder.h for
kernel code or defines the equivalent macros for userspace code.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I298e4d1dd224ffbb395e0df531b382a8c9e2082f
Reviewed-on: http://review.whamcloud.com/11986
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>