readtokens: avoid core dumps with unusual calling patterns

readtokens: avoid core dumps with unusual calling patterns

Reported by Xu Zhongxing in <http://debbugs.gnu.org/10953>.
* lib/readtokens.c: Include limits.h.
(word, bits_per_word, get_nth_bit, set_nth_bit): New.
(readtoken): Don't cache the delimiters; the cache code was buggy
if !delim && saved_delim, or if the new n_delim differs from the old.
Also, it wasn't thread-safe.