Merge branch '3499-duration-c-66-6-warning-array-subscript-is-of-type-char-on-netbsd-9' into 'main'

Resolve "duration.c:66:6: warning: array subscript is of type 'char' on NetBSD 9"

Closes #3499

See merge request isc-projects/bind9!6685

lib/isccfg/duration.c

@@ -63,7 +63,7 @@ isccfg_duration_fromtext(isc_textregion_t *source,
 	duration->unlimited = false;
 
 	/* Every duration starts with 'P' */
-	if (toupper(str[0]) != 'P') {
+	if (toupper((unsigned char)str[0]) != 'P') {
 		return (ISC_R_BADNUMBER);
 	}
 	P = str;