when given "dd -conv bogus ..." don't say "dd: bad arg -conv" say
"dd: bad conv bogus".

Reference: /n/sources/patch/applied/dd-errmsg
Date: Fri Jun 29 18:40:33 CES 2007
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/dd.c	Fri Jun 29 18:38:03 2007
+++ /sys/src/cmd/dd.c	Fri Jun 29 18:37:58 2007
@@ -170,6 +170,8 @@
 				cflag |= SYNC;
 				goto cloop;
 			}
+			fprint(2, "dd: bad conv %s\n", argv[c]);
+			exits("arg");
 		}
 		fprint(2, "dd: bad arg: %s\n", key);
 		exits("arg");