Eliminate C++ warning about deprecated register keyword (cord)

Eliminate C++ warning about deprecated register keyword (cord)

Issue #206 (bdwgc).

* cord/cordbscs.c (CORD_dump_inner, CORD_cat_char_star, CORD_cat,
CORD_from_fn_inner, CORD_index_access_fn, CORD_apply_access_fn,
CORD_substr_closure, CORD_substr_checked, CORD_substr, CORD_iter5,
CORD_riter4, CORD_init_min_len, CORD_init_forest, CORD_add_forest,
CORD_concat_forest, CORD_balance_insert, CORD_balance,
CORD__extend_path, CORD__pos_fetch, CORD__next, CORD__prev,
CORD_pos_fetch: Remove register keyword for local variables (and
function parameters).
* cord/cordprnt.c (extract_conv_spec, CORD_vsprintf): Likewise.
* cord/cordxtra.c (CORD_cat_char, CORD_catn, CORD_fill_proc,
CORD_batched_fill_proc, CORD_cmp, CORD_ncmp, CORD_to_char_star,
CORD_put_proc, CORD_batched_put_proc, CORD_chr_proc, CORD_rchr_proc,
CORD_batched_chr_proc, CORD_str, CORD_ec_flush_buf,
CORD_from_file_eager, refill_cache, CORD_lf_func,
CORD_from_file_lazy_inner, CORD_from_file_lazy, CORD_from_file):
Likewise.
* cord/tests/cordtest.c (test_basics, test_extras): Likewise.
* cord/tests/de.c (retrieve_line, redisplay): Likewise.
* cord/tests/de_win.c (plain_chars, control_chars, WndProc): Likewise.