Skip file or archive member if its transformed name is empty.

* NEWS: Document changes.
* doc/tar.texi: Document changes.
* src/common.h (transform_stat_info): Change return value.
(transform_name_fp): Change signature.
(WARN_EMPTY_TRANSFORM): New constant.
* src/create.c: Check return from transform_name.  Skip file, if it
is false.
* src/list.c (transform_stat_info): Return bool.
(read_and): Skip member if transform_stat_info returns false.
* src/transform.c (_transform_name_to_obstack): Change return type.
Always allocate result in obstack.
(transform_name_fp): Change arguments.  Return true on
success (transformed string not empty).  Otherwise return false and
don't change the source string.
* src/warning.c: New warning class: empty-transform.
* tests/extrac17.at: Use --warning=empty-transform.

tests/extrac17.at

@@ -38,7 +38,9 @@ genfile --file dir/subdir2/file2
 
 tar cf dir.tar dir
 
-tar -x -v -f dir.tar -C out --strip-components=2 --warning=no-timestamp \
+tar -x -v -f dir.tar -C out --strip-components=2 \
+  --warning=no-empty-transform \
+  --warning=no-timestamp \
   dir/subdir1/
 ],
 [0],