Enable conv=fdatasync in dd.

Matches behavior of coreutils dd.  Also removes unsupported
conversions.

Change-Id: I9ed5a7d43b5b083041b261ce70f6c835b41dc7ac
diff --git a/toolbox/dd.h b/toolbox/dd.h
index cca1024..89f2833 100644
--- a/toolbox/dd.h
+++ b/toolbox/dd.h
@@ -91,3 +91,4 @@
 #define	C_UNBLOCK	0x80000
 #define	C_OSYNC		0x100000
 #define	C_SPARSE	0x200000
+#define	C_FDATASYNC	0x400000