Improve command line option handling.

* gnulib.modules: Add argp-version-etc and progname,
use getopt-gnu instead of getopt.
* src/tar.c (HANG_OPTION, USAGE_OPTION)
(VERSION_OPTION): Remove.
(options): Remove corresponding options. Let argp
handle them.
(parse_opt): Likewise.
(_argp_hang): Removed.
(tar_authors): New variable.
(decode_options): Call argp_version_setup.
Do not use ARGP_NO_HELP flag in the call to argp_parse.