tar: remove lint

This lint was found by GCC 4.6.2 on Fedora 15 x86-64.
* src/buffer.c (buffer_write_global_xheader, mv_end, set_start_time)
(compute_duration, print_total_stats, flush_read, flush_write):
* src/checkpoint.c (checkpoint_finish_compile):
* src/list.c (test_archive_label):
* src/misc.c (chdir_count):
* src/names.c (const):
* src/unlink.c (finish_deferred_unlinks):
Define with (void) instead of with (), for slightly-better C type
checking and to avoid a GCC warning.
* src/compare.c (diff_dumpdir):
* src/tar.c (parse_owner_group): Remove unused local.
* src/misc.c (chdir_do):
* src/tar.c (add_exclude_array): Rename local to avoid shadowing.
(LOW_DENSITY_NUM, MID_DENSITY_NUM, HIGH_DENSITY_NUM):
Define only if needed.
* src/update.c (update_archive): Initialize a local; this fixes
what appears to be a real bug.