sort -df oddly assumes rune 0xff applies to all larger runes.  compute
the actual key.

Reference: /n/sources/patch/maybe/sort-df-rune
Date: Thu Mar 13 16:33:40 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/sort.c	Thu Mar 13 16:01:00 2014
+++ /sys/src/cmd/sort.c	Thu Mar 13 16:00:59 2014
@@ -1292,14 +1292,14 @@
 		/*
 		 * for characters out of range,
 		 * the table does not do Rflag.
-		 * ignore is based on mapto[nelem(f->mapto)-1]
+		 * ignore is computed directly.
 		 */
 		if(c != 0 && c < nelem(f->mapto)) {
 			c = f->mapto[c];
 			if(c == 0)
 				continue;
 		} else {
-			if(f->mapto[nelem(f->mapto)-1] == 0)
+			if(f->flags & Iflag)
 				continue;
 			/*
 			 * consider building maps as necessary