Implement shm_attach_to_open_removed() for OpenBSD

Implement shm_attach_to_open_removed() for OpenBSD

Add implementation which had been left by
0cfe2089 (Add runtime handlers for 97900ebf for FreeBSD/DragonFlyBSD).

This implementation is based on what
97900ebf (shm: have os remove shared memory if fio dies unexpectedly)
says. There seems to be no easy way to statically test versions like
KERNEL_VERSION() in Linux kernel.

The code assumes major/minor are both < 10. The minor has never been
10 or above. The major won't be 10 anytime soon based on the past
release cycle (current version is 6.1 after more than 2 decades).
https://en.wikipedia.org/wiki/OpenBSD_version_history

--
 # uname
 OpenBSD
 # uname -r
 6.1
 # cat ./test1.c
 #include <stdio.h>
 #include "./os/os.h"
 #include "./os/os-openbsd.h"
 int main(void) {
         printf("%d\n", shm_attach_to_open_removed());
         return 0;
 }
 # gcc -Wall -g ./test1.c
 # ./a.out
 1

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