Separate io_u from ioengine [1/3] - add io_u.h

Separate io_u from ioengine [1/3] - add io_u.h

Extract io_u related structures and functions from ioengine.h to
a new header io_u.h which is intended to be a header for io_u.c
(similar to the way there is io_u_queue.h for io_u_queue.c),
since there isn't much reason for them to be a part of ioengine.h.

This commit does nothing other than separating them out (and make
other related files include the new header).

Confirmed this compiles on Linux, Cygwin and several BSDs.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>