Allow for adding new members to Tar archives.

Allow for adding new members to Tar archives.

* lisp/tar-mode.el: Allow for adding new archive members.
(tar-new-regular-file-header, tar--pad-to, tar--put-at)
(tar-header-serialize): New functions.
(tar-current-position): Split from tar-current-descriptor.
(tar-current-descriptor): Use it.
(tar-new-entry): New command.
(tar-mode-map): Bind it.
* doc/emacs/files.texi (File Archives): Document "I" for tar-new-entry.
* etc/NEWS: Mention the new tar-new-entry command.

Fixes: debbugs:19274