tmpfiles: refactor code into new is_dir_empty function

The existing implementation seemed useful enough to warrant a new
helper function. Care was taken to ensure the revised code no longer
suffers any memory leaks.

src/tmpfiles.c

@@ -23,7 +23,6 @@
 
 #include "config.h"		/* Generated by configure script */
 
-#include <dirent.h>
 #include <ftw.h>
 #include <getopt.h>
 #include <glob.h>
@@ -394,17 +393,8 @@ static void tmpfiles(char *line)
 				paste(buf, sizeof(buf), "/usr/share/factory", path);
 				arg = buf;
 			}
-			if (!strc) {
-				struct dirent **namelist;
-				int num;
-
-				if (!S_ISDIR(st.st_mode))
-					break;
-				num = scandir(path, &namelist, NULL, NULL);
-				free(namelist);
-				if (num >= 3)
-					break; /* not empty */
-			}
+			if (fisdir(path) && !is_dir_empty(path))
+				break;
 			if (fisdir(arg) && !fisslashdir(arg)) {
 				size_t len = strlen(arg) + 2;