Automatic detection of seekable archives.

Automatic detection of seekable archives.

* src/buffer.c (guess_seekable_archive): New function.
(_open_archive): Call guess_seekable_archive for archives
open for reading.
(new_volume): Likewise.
* src/common.h (seek_option): New global.
* src/tar.c (options): New option --no-seek.
(parse_opt): --seek and --no-seek set seek_option,
not seekable_archive.
(decode_options): Initialize seek_option to -1.

* NEWS: Update.
* doc/tar.texi: Update.