convert to 4-byte UTF-8 and 32-bit Rune

http://codereview.appspot.com/116075
diff --git a/src/cmd/sed.c b/src/cmd/sed.c
index 5849c83..a78c9c6 100644
--- a/src/cmd/sed.c
+++ b/src/cmd/sed.c
@@ -615,7 +615,7 @@
 	while ((r = *cp++) != '\0') {
 		if(r == '\\') {
 			if (rhs < end)
-				*rhs++ = 0xFFFF;
+				*rhs++ = Runemax;
 			else
 				return 0;
 			r = *cp++;
@@ -1050,7 +1050,7 @@
 			sp = place(sp, loc1, loc2);
 			continue;
 		}
-		if (c == 0xFFFF && (c = *rp++) >= '1' && c < MAXSUB+'0') {
+		if (c == Runemax && (c = *rp++) >= '1' && c < MAXSUB+'0') {
 			n = c-'0';
 			if (subexp[n].s.rsp && subexp[n].e.rep) {
 				sp = place(sp, subexp[n].s.rsp, subexp[n].e.rep);