tar used to infer compression type from the filenames extension, but when
no file name is given (stdin/stdout), the -z flag was ignored and no
compression filter applied. this changes tar to assume the default
gzip compression method when z is given and no file name is specified.
this allows extracting tar archives that use longnames extension,
where the real filename is stored in a special entry with
linkflag == 'L' before the file entry. also skip longlink entries
with linkflag == 'K'.
