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?

doc/src/sgml/installation.sgml

@@ -874,7 +874,7 @@ su - postgres
        <listitem>
         <para>
          Build with Bonjour support.  This requires Bonjour support
-         in your operating system.  Recommended on OS X.
+         in your operating system.  Recommended on macOS.
         </para>
        </listitem>
       </varlistentry>
@@ -900,7 +900,7 @@ su - postgres
           <para>
            <option>e2fs</> to use the UUID library created by
            the <literal>e2fsprogs</> project; this library is present in most
-           Linux systems and in OS X, and can be obtained for other
+           Linux systems and in macOS, and can be obtained for other
            platforms as well
           </para>
          </listitem>
@@ -2010,7 +2010,7 @@ kill `cat /usr/local/pgsql/data/postmaster.pid`
   <para>
    <productname>PostgreSQL</> can be expected to work on these operating
    systems: Linux (all recent distributions), Windows (Win2000 SP4 and later),
-   FreeBSD, OpenBSD, NetBSD, OS X, AIX, HP/UX, Solaris,
+   FreeBSD, OpenBSD, NetBSD, macOS, AIX, HP/UX, Solaris,
    and UnixWare.  Other Unix-like systems may also work but are not currently
    being tested.  In most cases, all CPU architectures supported by
    a given operating system will work.  Look in