Make char unsigned, as busybox is designed for (and as ORCA/C does)

build

@@ -1,4 +1,4 @@
-cc -std=c99 \
+cc -std=c99 -funsigned-char \
 -Wall -Wno-format-security -Wno-comment -o hush \
 -Iinclude  -include include/autoconf.h \
 -Dhush_main=main -DNDEBUG  -D"BB_VER=KBUILD_STR(1.22.1)" -D"KBUILD_STR(s)=#s" \

shell/hush.c

@@ -5322,7 +5322,7 @@ static NOINLINE int expand_vars_to_list(o_string *output, int n, char *arg)
 			} else
 			/* If EXP_FLAG_SINGLEWORD, we handle assignment 'a=....$@.....'
 			 * and in this case should treat it like '$*' - see 'else...' below */
-			if ((unsigned char)first_ch == ('@'|0x80)  /* quoted $@ */
+			if (first_ch == ('@'|0x80)  /* quoted $@ */
 			 && !(output->o_expflags & EXP_FLAG_SINGLEWORD) /* not v="$@" case */
 			) {
 				while (1) {