MINOR: add list_append_word function

MINOR: add list_append_word function

int list_append_word(struct list *li, const char *str, char **err)

Append a copy of string <str> (inside a wordlist) at the end of
the list <li>.
The caller is responsible for freeing the <err> and <str> copy memory
area using free().

On failure : return 0 and <err> filled with an error message.