Add ioprio_set() support for DragonFlyBSD

Add ioprio_set() support for DragonFlyBSD

It basically has the same interface as Linux kernel's ioprio_set(2),
but needed to workaround a compile issue by defining it as a macro
as mentioned in a comment (it can be called without syscall(NR, ...)).

This commit is based on the last two commits which made I/O priority
option more generic for non Linux environment.

 # uname
 DragonFly
 # fio --cmdhelp | grep prioclass -B1
 prio                    : Set job IO priority value
 prioclass               : Your platform does not support IO priority classes

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>