Back-patch use of unnamed POSIX semaphores for Linux?

Just saw another report of what's probably systemd killing off Postgres'
SysV semaphores, as we've discussed previously at, eg,
https://www.postgresql.org/message-id/flat/57828C31.5060409%40gmail.com
Since the systemd people are generally impervious to suggestions that
they might be mistaken, I do not expect this problem to go away.

I think we should give serious consideration to back-patching commit
ecb0d20a9, which changed the default semaphore type to unnamed-POSIX
on Linux. We've seen no problems in the buildfarm in the two months
that that's been in HEAD. If we don't do this, we can expect to
continue seeing complaints of this sort until pre-v10 PG releases
fall out of use ... and I don't want to wait that long.

Commit ecb0d20a9 also changed the default for FreeBSD. I'm not convinced
we should back-patch that part, because (a) unnamed-POSIX semas have
only been there since FreeBSD 9.0, which isn't that long ago, and (b)
the argument for switching is "it'll perform better" not "your server
will fail randomly without this change".

Comments?

regards, tom lane