Proposed on the the tech-userlevel mailing list without strong objections.
This file no longer serves any purpose in any supported release branch.
There will be fallout in pkgsrc, please restrict inclusion <sys/user.h> to
FreeBSD and DragonflyBSD.
I tried to iterate over packages in pkgsrc and there was little fallout,
however I don't have the full sources to scan every source code. I already
upstreamed to several projects removal of inclusion on NetBSD of
<sys/user.h> (like Qt5).
bd6e82e7ba