Replace hand-rolled modulo with arc4random_uniform.

Fixes potential modulo-by-zero UB flagged by Coverity CID 405068

Author: daztucker

regress/netcat.c

@@ -1134,7 +1134,7 @@ build_ports(char *p)
 			char *c;
 
 			for (x = 0; x <= (hi - lo); x++) {
-				y = (arc4random() & 0xFFFF) % (hi - lo);
+				y = arc4random_uniform(hi - lo);
 				c = portlist[x];
 				portlist[x] = portlist[y];
 				portlist[y] = c;