Fix EXEC BACKEND + Windows builds for group privs

Under EXEC BACKEND we also need to be going through the group privileges
setup since we do support that on Unixy systems, so add that to
SubPostmasterMain().

Under Windows, we need to simply return true from
GetDataDirectoryCreatePerm(), but that wasn't happening due to a missing
 #else clause.

Per buildfarm.

src/backend/postmaster/postmaster.c

@@ -4801,6 +4801,14 @@ SubPostmasterMain(int argc, char *argv[])
 	/* Read in remaining GUC variables */
 	read_nondefault_variables();
 
+	/*
+	 * Check that the data directory looks valid, which will also check the
+	 * privileges on the data directory and update our umask and file/group
+	 * variables for creating files later.  Note: this should really be done
+	 * before we create any files or directories.
+	 */
+	checkDataDir();
+
 	/*
 	 * (re-)read control file, as it contains config. The postmaster will
 	 * already have read this, but this process doesn't know about that.

src/common/file_perm.c

@@ -78,9 +78,14 @@ GetDataDirectoryCreatePerm(const char *dataDir)
 
 	/* Set permissions */
 	SetDataDirectoryCreatePerm(statBuf.st_mode);
-
 	return true;
-#endif							/* !defined(WIN32) && !defined(__CYGWIN__) */
+#else							/* !defined(WIN32) && !defined(__CYGWIN__) */
+	/*
+	 * On Windows, we don't have anything to do here since they don't have
+	 * Unix-y permissions.
+	 */
+	return true;
+#endif
 }