Re: Let's drop support for named POSIX semaphores

+1 on dropping it.

Peter, as to your concern that this adds another hard SysV dependency
and so cuts against the work to let PostgreSQL run without SysV IPC (for
macOS sandboxing): I don't think removing *named* POSIX semaphores costs
that effort anything, because named POSIX semaphores are not the thing
that effort would reach for.

The non-SysV semaphore path we'd actually want is the *unnamed* kind
(sem_init), which posix_sema.c already prefers and which backs the sem_t
structs out of shared memory. The named kind (sem_open) is only the
fallback, and the header comment there spells out why it's a dead end
for this:

With unnamed semaphores, the sem_t structs live in an array in shared
memory [...] Therefore, the named-semaphore code *does not cope with
EXEC_BACKEND*.

So a SysV-free future built on POSIX semaphores runs through the unnamed
path Tom is keeping, not the named path he's removing. Dropping named
POSIX semaphores narrows the matrix to "SysV or unnamed-POSIX".

(+1 as well to Shihao's two nits, the now-unused IPCProtection macro
and the stale runtime.sgml line about which platforms use which.)

best,

-greg