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/backend/Makefile

@@ -26,8 +26,8 @@ include $(srcdir)/common.mk
 # As of 1/2010:
 # The probes.o file is necessary for dtrace support on Solaris, and on recent
 # versions of systemtap.  (Older systemtap releases just produce an empty
-# file, but that's okay.)  However, OS X's dtrace doesn't use it and doesn't
-# even recognize the -G option.  So, build probes.o except on Darwin.
+# file, but that's okay.)  However, macOS's dtrace doesn't use it and doesn't
+# even recognize the -G option.  So, build probes.o except on macOS.
 # This might need adjustment as other platforms add dtrace support.
 ifneq ($(PORTNAME), darwin)
 ifeq ($(enable_dtrace), yes)

src/backend/port/dynloader/darwin.c

@@ -1,5 +1,5 @@
 /*
- * Dynamic loading support for Darwin
+ * Dynamic loading support for macOS (Darwin)
  *
  * If dlopen() is available (Darwin 10.3 and later), we just use it.
  * Otherwise we emulate it with the older, now deprecated, NSLinkModule API.

src/backend/postmaster/postmaster.c

@@ -1273,7 +1273,7 @@ PostmasterMain(int argc, char *argv[])
 #ifdef HAVE_PTHREAD_IS_THREADED_NP
 
 	/*
-	 * On Darwin, libintl replaces setlocale() with a version that calls
+	 * On macOS, libintl replaces setlocale() with a version that calls
 	 * CFLocaleCopyCurrent() when its second argument is "" and every relevant
 	 * environment variable is unset or empty.  CFLocaleCopyCurrent() makes
 	 * the process multithreaded.  The postmaster calls sigprocmask() and

src/backend/utils/adt/varlena.c

@@ -1844,8 +1844,8 @@ varstr_sortsupport(SortSupport ssup, Oid collid, bool bpchar)
 	 * Even apart from the risk of broken locales, it's possible that there
 	 * are platforms where the use of abbreviated keys should be disabled at
 	 * compile time.  Having only 4 byte datums could make worst-case
-	 * performance drastically more likely, for example.  Moreover, Darwin's
-	 * strxfrm() implementations is known to not effectively concentrate a
+	 * performance drastically more likely, for example.  Moreover, macOS's
+	 * strxfrm() implementation is known to not effectively concentrate a
 	 * significant amount of entropy from the original string in earlier
 	 * transformed blobs.  It's possible that other supported platforms are
 	 * similarly encumbered.  So, if we ever get past disabling this

src/backend/utils/misc/ps_status.c

@@ -223,8 +223,8 @@ save_ps_display_args(int argc, char **argv)
 #if defined(__darwin__)
 
 		/*
-		 * Darwin (and perhaps other NeXT-derived platforms?) has a static
-		 * copy of the argv pointer, which we may fix like so:
+		 * macOS (and perhaps other NeXT-derived platforms?) has a static copy
+		 * of the argv pointer, which we may fix like so:
 		 */
 		*_NSGetArgv() = new_argv;
 #endif

src/backend/utils/probes.d

@@ -12,7 +12,7 @@
  * Typedefs used in PostgreSQL.
  *
  * NOTE: Do not use system-provided typedefs (e.g. uintptr_t, uint32_t, etc)
- * in probe definitions, as they cause compilation errors on Mac OS X 10.5.
+ * in probe definitions, as they cause compilation errors on macOS 10.5.
  */
 #define LocalTransactionId unsigned int
 #define LWLockMode int