pg_upgrade: fix CopyFile() on Windows to fail on file existence

Also fix getErrorText() to return the right error string on failure.
This behavior now matches that of other operating systems.

Report by Noah Misch

Backpatch through 9.1

contrib/pg_upgrade/file.c

@@ -41,7 +41,7 @@ copyAndUpdateFile(pageCnvCtx *pageConverter,
 #ifndef WIN32
 		if (copy_file(src, dst, force) == -1)
 #else
-		if (CopyFile(src, dst, force) == 0)
+		if (CopyFile(src, dst, !force) == 0)
 #endif
 			return getErrorText(errno);
 		else

contrib/pg_upgrade/util.c

@@ -229,6 +229,8 @@ getErrorText(int errNum)
 {
 #ifdef WIN32
 	_dosmaperr(GetLastError());
+	/* _dosmaperr sets errno, so we copy errno here */
+	errNum = errno;
 #endif
 	return strdup(strerror(errNum));
 }