Replace sftp_pkt_get* with BinarySource.

This is the first major piece of code converted to the new
unmarshalling system, and allows me to remove all the sftp_pkt_get*
functions in sftp.c that were previously duplicating standard decode
logic.

sftp.h

@@ -2,6 +2,7 @@
  * sftp.h: definitions for SFTP and the sftp.c routines.
  */
 
+#include "defs.h"
 #include "int64.h"
 
 #define SSH_FXP_INIT                              1	/* 0x1 */