(ITS#8986) [PATCH] Fix union semun undefined from FreeBSD 12 onward

Full_Name: Ka Ho Ng Version: mdb.master OS: FreeBSD 12.0-RELEASE URL: ftp://ftp.openldap.org/incoming/Ka-Ho-Ng-190306.patch Submission from: (NULL) (2001:470:fa95:1300::1)

Starting from __FreeBSD_version 1200059 union semun definition was removed from userspace headers in order to comply with POSIX. In order to resurrect the defintion we need to define _WANT_SEMUN before including sys/sem.h for __FreeBSD_version >= 1200059.

I tried to touch as small amount of code as possible here to avoid interfering with other platforms I do not use.