Fix problems caught by static checking

Fix problems caught by static checking

Most of these can be caught by configuring with --enable-gcc-warnings.
Problem reported by Pavel Raiskup in:
http://lists.gnu.org/archive/html/bug-tar/2015-09/msg00001.html
* src/buffer.c (format_total_stats):
Prefer pointer to const when it’s OK to use const.
(default_total_format): Now const.
* src/buffer.c (default_total_format):
* src/exclist.c (excfile_head, excfile_tail, vcs_ignore_files):
* src/misc.c (namebuf_add_dir, namebuf_finish):
* src/tar.c (verbatim_files_from_option, option_set_in_cl)
(optloc_eq, set_old_files_option):
Now static.
* src/common.h: Adjust to match the other changes described here.
* src/exclist.c (info_cleanup_exclist):
* src/tar.c (argp_program_version, argp_program_bug_address):
Remove; unused.
(parse_default_options): Define via prototype instead of old style.
(parse_default_options, decode_options):
Fill out incomplete initializers.