Fix ctype(3) abuse

[iamleot]

isascii(3), isspace(3), isupper(3), tolower(3) and toupper(3) functions only accepts values that are representable as unsigned char or is EOF. For all other values the behavior is undefined.

Cast to unsigned char to avoid any undefined behavior.

---

This was noticed on NetBSD.

---

Trying to build commit a61f178 in NetBSD/amd64 -current (11.99.1) we have the following compiler warnings:

$ ./build.sh
[...]
--- src/ignore.o ---
In file included from /usr/include/ctype.h:100,
                 from src/ignore.c:1:
src/ignore.c: In function ‘add_ignore_pattern’:
src/ignore.c:110:29: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  110 |         if (!isspace(pattern[pattern_len - 1])) {
      |                             ^
[...]
--- src/util.o ---
  CC       src/util.o
In file included from /usr/include/ctype.h:100,
                 from src/util.c:1:
src/util.c: In function ‘generate_alpha_skip’:
src/util.c:82:52: warning: array subscript has type ‘char’ [-Wchar-subscripts]
   82 |             skip_lookup[(unsigned char)tolower(find[i])] = f_len - i;
      |                                                    ^
src/util.c:83:52: warning: array subscript has type ‘char’ [-Wchar-subscripts]
   83 |             skip_lookup[(unsigned char)toupper(find[i])] = f_len - i;
      |                                                    ^
src/util.c: In function ‘is_prefix’:
src/util.c:97:26: warning: array subscript has type ‘char’ [-Wchar-subscripts]
   97 |             if (tolower(s[i]) != tolower(s[i + pos])) {
      |                          ^
src/util.c:97:43: warning: array subscript has type ‘char’ [-Wchar-subscripts]
   97 |             if (tolower(s[i]) != tolower(s[i + pos])) {
      |                                           ^
src/util.c: In function ‘suffix_len’:
src/util.c:115:26: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  115 |             if (tolower(s[pos - i]) != tolower(s[s_len - i - 1])) {
      |                          ^
src/util.c:115:49: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  115 |             if (tolower(s[pos - i]) != tolower(s[s_len - i - 1])) {
      |                                                 ^
src/util.c: In function ‘boyer_moore_strnstr’:
src/util.c:193:68: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  193 |         for (i = f_len - 1; i >= 0 && (case_insensitive ? tolower(s[pos]) : s[pos]) == find[i]; pos--, i--) {
      |                                                                    ^
src/util.c: In function ‘hash_strnstr’:
src/util.c:220:55: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  220 |                 if ((case_sensitive ? R[i] : tolower(R[i])) != find[i])
      |                                                       ^
src/util.c:232:57: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  232 |             char s_c = case_sensitive ? R[i] : tolower(R[i]);
      |                                                         ^
src/util.c: In function ‘is_lowercase’:
src/util.c:462:40: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  462 |         if (!isascii(s[i]) || isupper(s[i])) {
      |                                        ^
[...]
--- src/main.o ---
  CC       src/main.o
In file included from /usr/include/ctype.h:100,
                 from src/main.c:1:
src/main.c: In function ‘main’:
src/main.c:120:36: warning: array subscript has type ‘char’ [-Wchar-subscripts]
  120 |                 *c = (char)tolower(*c);
      |                                    ^

For possible more information regarding ctype(3) abuse please give a look to CAVEATS section of ctype(3) NetBSD man page.