tweaks: make sure calls to <ctype.h> functions/macros use "unsigned char"

The platform's default char type might be signed, which could cause
problems in 8-bit locales.

This addresses https://savannah.gnu.org/bugs/?50289

.
Reported-by: Hans-Bernhard Broeker <HBBroeker@T-Online.de>

src/chars.c

@@ -736,7 +736,7 @@ char *mbrevstrpbrk(const char *s, const char *accept, const char
 bool has_blank_chars(const char *s)
 {
     for (; *s != '\0'; s++) {
-	if (isblank(*s))
+	if (isblank((unsigned char)*s))
 	    return TRUE;
     }
 

src/global.c

@@ -424,7 +424,7 @@ void assign_keyinfo(sc *s, const char *keystring, const int keycode)
 	if (strcasecmp(keystring, "M-Space") == 0)
 	    s->keycode = (int)' ';
 	else
-	    s->keycode = tolower((int)keystring[2]);
+	    s->keycode = tolower((unsigned char)keystring[2]);
     } else if (keystring[0] == 'F')
 	s->keycode = KEY_F0 + atoi(&keystring[1]);
     else if (!strcasecmp(keystring, "Ins"))

src/rcfile.c

@@ -167,7 +167,7 @@ void rcfile_error(const char *msg, ...)
  * returned pointer will point to '\0' if we hit the end of the line. */
 char *parse_next_word(char *ptr)
 {
-    while (!isblank(*ptr) && *ptr != '\0')
+    while (!isblank((unsigned char)*ptr) && *ptr != '\0')
 	ptr++;
 
     if (*ptr == '\0')
@@ -176,7 +176,7 @@ char *parse_next_word(char *ptr)
     /* Null-terminate and advance ptr. */
     *ptr++ = '\0';
 
-    while (isblank(*ptr))
+    while (isblank((unsigned char)*ptr))
 	ptr++;
 
     return ptr;
@@ -215,7 +215,7 @@ char *parse_argument(char *ptr)
 	ptr = last_quote + 1;
     }
     if (ptr != NULL)
-	while (isblank(*ptr))
+	while (isblank((unsigned char)*ptr))
 	    ptr++;
     return ptr;
 }
@@ -230,7 +230,7 @@ char *parse_next_regex(char *ptr)
     /* Continue until the end of line, or until a " followed by a
      * blank character or the end of line. */
     while (*ptr != '\0' && (*ptr != '"' ||
-		(*(ptr + 1) != '\0' && !isblank(*(ptr + 1)))))
+		(*(ptr + 1) != '\0' && !isblank((unsigned char)ptr[1]))))
 	ptr++;
 
     assert(*ptr == '"' || *ptr == '\0');
@@ -244,7 +244,7 @@ char *parse_next_regex(char *ptr)
     /* Null-terminate and advance ptr. */
     *ptr++ = '\0';
 
-    while (isblank(*ptr))
+    while (isblank((unsigned char)*ptr))
 	ptr++;
 
     return ptr;
@@ -387,11 +387,11 @@ void parse_binding(char *ptr, bool dobind)
     }
 
     /* Uppercase only the first two or three characters of the key name. */
-    keycopy[0] = toupper(keycopy[0]);
-    keycopy[1] = toupper(keycopy[1]);
+    keycopy[0] = toupper((unsigned char)keycopy[0]);
+    keycopy[1] = toupper((unsigned char)keycopy[1]);
     if (keycopy[0] == 'M' && keycopy[1] == '-') {
 	if (strlen(keycopy) > 2)
-	    keycopy[2] = toupper(keycopy[2]);
+	    keycopy[2] = toupper((unsigned char)keycopy[2]);
 	else {
 	    rcfile_error(N_("Key name is too short"));
 	    goto free_copy;
@@ -401,7 +401,7 @@ void parse_binding(char *ptr, bool dobind)
     /* Allow the codes for Insert and Delete to be rebound, but apart
      * from those two only Control, Meta and Function sequences. */
     if (!strcasecmp(keycopy, "Ins") || !strcasecmp(keycopy, "Del"))
-	keycopy[1] = tolower(keycopy[1]);
+	keycopy[1] = tolower((unsigned char)keycopy[1]);
     else if (keycopy[0] != '^' && keycopy[0] != 'M' && keycopy[0] != 'F') {
 	rcfile_error(N_("Key name must begin with \"^\", \"M\", or \"F\""));
 	goto free_copy;
@@ -966,7 +966,7 @@ void parse_rcfile(FILE *rcstream, bool syntax_only)
 
 	lineno++;
 	ptr = buf;
-	while (isblank(*ptr))
+	while (isblank((unsigned char)*ptr))
 	    ptr++;
 
 	/* If we have a blank line or a comment, skip to the next