Support >INT_MAX -C dirs

Support >INT_MAX -C dirs

* src/extract.c (struct delayed_set_stat, struct delayed_link):
* src/misc.c (normalize_filename, wd_count, chdir_count)
(chdir_arg, tar_getcdpath):
* src/names.c (name_gather, addname, add_hierarchy_to_namelist):
* src/unlink.c (struct deferred_unlink, flush_deferred_unlinks):
Use idx_t, not int, for directory indexes, so as to not
limit their number to INT_MAX; this is theoretically possible
if -T is used.
* src/names.c (name_next_elt, name_next):
Use bool for boolean.