Refine win32env.c cosmetics.

Replace use of plain 0 as a null pointer constant.  In comments, update
terminology and lessen redundancy.  Back-patch to 9.2 (all supported
versions) for the convenience of back-patching the next two commits.

Christian Ullrich and Noah Misch, reviewed (in earlier versions) by
Michael Paquier.

src/port/win32env.c

 /*-------------------------------------------------------------------------
  *
  * win32env.c
- *	  putenv() and unsetenv() for win32, that updates both process
- *	  environment and the cached versions in (potentially multiple)
- *	  MSVCRT.
+ *	  putenv() and unsetenv() for win32, which update both process environment
+ *	  and caches in (potentially multiple) C run-time library (CRT) versions.
  *
  * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
@@ -24,17 +23,10 @@ pgwin32_putenv(const char *envval)
 	char	   *cp;
 
 	/*
-	 * Each version of MSVCRT has its own _putenv() call in the runtime
-	 * library.
-	 *
-	 * mingw always uses MSVCRT.DLL, but if we are in a Visual C++
-	 * environment, attempt to update the environment in all MSVCRT modules
-	 * that are currently loaded, to work properly with any third party
-	 * libraries linked against a different MSVCRT but still relying on
-	 * environment variables.
-	 *
-	 * Also separately update the system environment that gets inherited by
-	 * subprocesses.
+	 * Each CRT has its own _putenv() symbol and copy of the environment.
+	 * Update the environment in each CRT module currently loaded, so every
+	 * third-party library sees this change regardless of the CRT it links
+	 * against.
 	 */
 #ifdef _MSC_VER
 	typedef int (_cdecl * PUTENVPROC) (const char *);
@@ -46,34 +38,34 @@ pgwin32_putenv(const char *envval)
 	}			rtmodules[] =
 	{
 		{
-			"msvcrt", 0, NULL
-		},						/* Visual Studio 6.0 / mingw */
+			"msvcrt", NULL, NULL
+		},						/* Visual Studio 6.0 / MinGW */
 		{
-			"msvcr70", 0, NULL
+			"msvcr70", NULL, NULL
 		},						/* Visual Studio 2002 */
 		{
-			"msvcr71", 0, NULL
+			"msvcr71", NULL, NULL
 		},						/* Visual Studio 2003 */
 		{
-			"msvcr80", 0, NULL
+			"msvcr80", NULL, NULL
 		},						/* Visual Studio 2005 */
 		{
-			"msvcr90", 0, NULL
+			"msvcr90", NULL, NULL
 		},						/* Visual Studio 2008 */
 		{
-			"msvcr100", 0, NULL
+			"msvcr100", NULL, NULL
 		},						/* Visual Studio 2010 */
 		{
-			"msvcr110", 0, NULL
+			"msvcr110", NULL, NULL
 		},						/* Visual Studio 2012 */
 		{
-			"msvcr120", 0, NULL
+			"msvcr120", NULL, NULL
 		},						/* Visual Studio 2013 */
 		{
-			"ucrtbase", 0, NULL
+			"ucrtbase", NULL, NULL
 		},						/* Visual Studio 2015 and later */
 		{
-			NULL, 0, NULL
+			NULL, NULL, NULL
 		}
 	};
 	int			i;
@@ -82,7 +74,7 @@ pgwin32_putenv(const char *envval)
 	{
 		if (rtmodules[i].putenvFunc == NULL)
 		{
-			if (rtmodules[i].hmodule == 0)
+			if (rtmodules[i].hmodule == NULL)
 			{
 				/* Not attempted before, so try to find this DLL */
 				rtmodules[i].hmodule = GetModuleHandle(rtmodules[i].modulename);
@@ -121,8 +113,8 @@ pgwin32_putenv(const char *envval)
 #endif   /* _MSC_VER */
 
 	/*
-	 * Update the process environment - to make modifications visible to child
-	 * processes.
+	 * Update process environment, making this change visible to child
+	 * processes and to CRTs initializing in the future.
 	 *
 	 * Need a copy of the string so we can modify it.
 	 */
@@ -142,7 +134,7 @@ pgwin32_putenv(const char *envval)
 		/*
 		 * Only call SetEnvironmentVariable() when we are adding a variable,
 		 * not when removing it. Calling it on both crashes on at least
-		 * certain versions of MingW.
+		 * certain versions of MinGW.
 		 */
 		if (!SetEnvironmentVariable(envcpy, cp))
 		{