Issue a warning if the archive being compared contais transformed file names.

* src/common.h (transform_program_p): New proto.
* src/transform.c (transform_program_p): New function.
* src/compare.c (verify_volume): Warn if the archive contains
transformed file names.

src/transform.c

@@ -628,3 +628,9 @@ transform_name (char **pinput, int type)
 {
   return transform_name_fp (pinput, type, NULL, NULL);
 }
+
+bool
+transform_program_p (void)
+{
+  return transform_head != NULL;
+}