dc: explicitly declare signed char for number buffers

Parts of the code depend on char being signed.

Showed up for me trying square root, but probably affects other stuff
too.

Applications/V7/cmd/dc.c

@@ -45,10 +45,10 @@
 #define errorrt(p)	{printf(p); return NULL; }
 
 struct blk {
-	char *rd;
-	char *wt;
-	char *beg;
-	char *last;
+	signed char *rd;
+	signed char *wt;
+	signed char *beg;
+	signed char *last;
 };
 struct sym {
 	struct sym *next;
@@ -145,8 +145,8 @@ void (*outdit) (struct blk *, int);
 int logo;
 int intlog10;
 int count;
-char *pp;
-char *dummy;
+signed char *pp;
+signed char *dummy;
 
 int main(int argc, char *argv[])
 {
@@ -1996,7 +1996,7 @@ void sdump(char *s1, struct blk *hptr)
 
 void seekc(struct blk *hptr, int n)
 {
-	char *nn, *p;
+	signed char *nn, *p;
 
 	nn = hptr->beg + n;
 	if (nn > hptr->last) {
@@ -2123,7 +2123,7 @@ void garbage(char *s)
 void redef(struct blk *p)
 {
 	register int offset;
-	register char *newp;
+	register signed char *newp;
 
 	if ((int) p->beg & 01) {
 		printf("odd ptr %o hdr %o\n", p->beg, p);