Skip some permissions checks on Cygwin

These are checks that are already skipped on other Windows systems.

Backpatch to all live branches, as appropriate.

src/bin/initdb/t/001_initdb.pl

@@ -80,7 +80,7 @@ command_fails([ 'initdb', $datadir ], 'existing data directory');
 SKIP:
 {
 	skip "unix-style permissions not supported on Windows", 2
-	  if ($windows_os);
+	  if ($windows_os || $Config::Config{osname} eq 'cygwin');
 
 	# Init a new db with group access
 	my $datadir_group = "$tempdir/data_group";

src/bin/pg_basebackup/t/010_pg_basebackup.pl

@@ -489,7 +489,7 @@ SKIP:
 SKIP:
 {
 	skip "unix-style permissions not supported on Windows", 1
-	  if ($windows_os);
+	  if ($windows_os || $Config::Config{osname} eq 'cygwin');
 
 	ok(check_mode_recursive("$tempdir/backup1", 0750, 0640),
 		"check backup dir permissions");

src/bin/pg_ctl/t/001_start_stop.pl

@@ -79,7 +79,8 @@ $logFileName = "$tempdir/data/perm-test-640.log";
 
 SKIP:
 {
-	skip "group access not supported on Windows", 3 if ($windows_os);
+	skip "group access not supported on Windows", 3
+	  if ($windows_os || $Config::Config{osname} eq 'cygwin');
 
 	system_or_bail 'pg_ctl', 'stop', '-D', "$tempdir/data";
 

src/bin/pg_rewind/t/002_databases.pl

@@ -60,7 +60,7 @@ template1
   SKIP:
 	{
 		skip "unix-style permissions not supported on Windows", 1
-		  if ($windows_os);
+		  if ($windows_os || $Config::Config{osname} eq 'cygwin');
 
 		ok(check_mode_recursive($node_primary->data_dir(), 0750, 0640),
 			'check PGDATA permissions');

src/bin/pg_verifybackup/t/003_corruption.pl

@@ -101,7 +101,8 @@ for my $scenario (@scenario)
   SKIP:
 	{
 		skip "unix-style permissions not supported on Windows", 4
-		  if $scenario->{'skip_on_windows'} && $windows_os;
+		  if ($scenario->{'skip_on_windows'} &&
+			  ($windows_os || $Config::Config{osname} eq 'cygwin'));
 
 		# Take a backup and check that it verifies OK.
 		my $backup_path    = $primary->backup_dir . '/' . $name;