(tar-mode): Position point on the name of the first file.

(tar-mode): Position point on the name of the first file.
(tar-extract): Detect coding-system of the archive member and
decode it like insert-file-contents does.
(tar-alter-one-field): Reposition point on the file name of the
current tar entry.
(tar-subfile-save-buffer): Encode the file when updating it in the
archive, and use the size of encoded text to update the header
block.  Set last-coding-system-used to coding-system of the file.
Restore point of tar-superior-buffer after updating the descriptor line.