dd(1): Use a local swapbytes() function.

swab(3) has restrict qualifiers for src and dst.
Avoid relying on undefined overlapping swab behavior.

Obtained From: OpenBSD

bin/dd/dd.c

@@ -338,6 +338,21 @@ speed_limit(void)
 	t_prev = t_now;
 }
 
+static void
+swapbytes(void *v, size_t len)
+{
+	unsigned char *p = v;
+	unsigned char t;
+
+	while (len > 1) {
+		t = p[0];
+		p[0] = p[1];
+		p[1] = t;
+		p += 2;
+		len -= 2;
+	}
+}
+
 static void
 dd_in(void)
 {
@@ -438,7 +453,7 @@ dd_in(void)
 				++st.swab;
 				--n;
 			}
-			swab(in.dbp, in.dbp, (size_t)n);
+			swapbytes(in.dbp, (size_t)n);
 		}
 
 		in.dbp += in.dbrcnt;