Refer to OS X as "macOS", except for the port name which is still "darwin".

We weren't terribly consistent about whether to call Apple's OS "OS X"
or "Mac OS X", and the former is probably confusing to people who aren't
Apple users.  Now that Apple has rebranded it "macOS", follow their lead
to establish a consistent naming pattern.  Also, avoid the use of the
ancient project name "Darwin", except as the port code name which does not
seem desirable to change.  (In short, this patch touches documentation and
comments, but no actual code.)

I didn't touch contrib/start-scripts/osx/, either.  I suspect those are
obsolete and due for a rewrite, anyway.

I dithered about whether to apply this edit to old release notes, but
those were responsible for quite a lot of the inconsistencies, so I ended
up changing them too.  Anyway, Apple's being ahistorical about this,
so why shouldn't we be?

src/template/darwin

@@ -1,9 +1,12 @@
 # src/template/darwin
 
-# Select appropriate semaphore support.  Darwin 6.0 (Mac OS X 10.2) and up
-# support System V semaphores; before that we have to use POSIX semaphores,
-# which are less good for our purposes because they eat a file descriptor
-# per backend per max_connection slot.
+# Note: Darwin is the original code name for macOS, also known as OS X.
+# We still use "darwin" as the port name, partly because config.guess does.
+
+# Select appropriate semaphore support.  Darwin 6.0 (macOS 10.2) and up
+# support System V semaphores; before that we have to use named POSIX
+# semaphores, which are less good for our purposes because they eat a
+# file descriptor per backend per max_connection slot.
 case $host_os in
   darwin[015].*)
     USE_NAMED_POSIX_SEMAPHORES=1