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.

src/misc.c

@@ -29,6 +29,8 @@
 # define DOUBLE_SLASH_IS_DISTINCT_ROOT 0
 #endif
 
+static void namebuf_add_dir (namebuf_t, char const *);
+static char *namebuf_finish (namebuf_t);
 static const char *tar_getcdpath (int);
 
 
@@ -1202,7 +1204,7 @@ namebuf_name (namebuf_t buf, const char *name)
   return buf->buffer;
 }
 
-void
+static void
 namebuf_add_dir (namebuf_t buf, const char *name)
 {
   static char dirsep[] = { DIRECTORY_SEPARATOR, 0 };
@@ -1215,7 +1217,7 @@ namebuf_add_dir (namebuf_t buf, const char *name)
   buf->dir_length += strlen (name);
 }
 
-char *
+static char *
 namebuf_finish (namebuf_t buf)
 {
   char *res = buf->buffer;