Fix tar-mode reading the oldgnu Tar format

Fix tar-mode reading the oldgnu Tar format

This makes sure an entry for a long file name will not
accidentally appear as a directory, and thus its size will be
disregarded, causing corrupted file headers for all the subsequent
entries.  The original problem happened because the long file name
truncated to 99 bytes happened to end in a slash, which tar-mode
takes to be the indication of a directory.
* lisp/tar-mode.el (tar-header-block-tokenize): Accept an
additional argument DISABLE-SLASH; if non-nil, don't set the
link-type field of the descriptor to 5 (meaning a directory) just
because the name ends in a slash.  Use this argument when calling
itself recursively, to read the entry of the file with a long
name.  Set the link-type to 5 if the long name ends in a slash.
(Bug#38777)