core: warn if a generator is world-writable

... because that is obviously a security risk.

src/shared/exec-util.h

@@ -20,6 +20,7 @@ typedef enum {
         EXEC_DIR_IGNORE_ERRORS        = 1 << 1, /* Ignore non-zero exit status of scripts */
         EXEC_DIR_SET_SYSTEMD_EXEC_PID = 1 << 2, /* Set $SYSTEMD_EXEC_PID environment variable */
         EXEC_DIR_SKIP_REMAINING       = 1 << 3, /* Ignore remaining executions when one exit with 77. */
+        EXEC_DIR_WARN_WORLD_WRITABLE  = 1 << 4, /* Warn if world writable files are found */
 } ExecDirFlags;
 
 typedef enum ExecCommandFlags {