argp: Don't pass invalid arguments to isspace, isalnum, isalpha, isdigit.

* lib/argp-help.c (SKIPWS): Cast character to 'unsigned char' before passing it
to isspace().
(fill_in_uparams): Likewise for isalpha(), isalnum(), isdigit().
(canon_doc_option): Likewise for isspace(), isalnum().

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>