LIBSSH2_SFTP_PACKET_MAXLEN: increase to 80000

Some SFTP servers send SFTP packets larger than 40000. Since the limit
is only present to avoid insane sizes anyway, we can easily bump it.

The define was formerly in the public header libssh2_sftp.h but served
no external purpose and was moved into the source dir.

Bug: http://www.libssh2.org/mail/libssh2-devel-archive-2011-11/0004.shtml
Reported by: Michael Harris

include/libssh2_sftp.h

@@ -54,7 +54,6 @@ extern "C" {
  * Let's start with Version 3 (The version found in OpenSSH) and go from there
  */
 #define LIBSSH2_SFTP_VERSION        3
-#define LIBSSH2_SFTP_PACKET_MAXLEN  40000
 
 typedef struct _LIBSSH2_SFTP                LIBSSH2_SFTP;
 typedef struct _LIBSSH2_SFTP_HANDLE         LIBSSH2_SFTP_HANDLE;

src/sftp.c

@@ -89,6 +89,10 @@
 #define SSH_FXE_STATVFS_ST_RDONLY               0x00000001
 #define SSH_FXE_STATVFS_ST_NOSUID               0x00000002
 
+/* This is the maximum packet length to accept, as larger than this indicate
+   some kind of server problem. */
+#define LIBSSH2_SFTP_PACKET_MAXLEN  80000
+
 static int sftp_close_handle(LIBSSH2_SFTP_HANDLE *handle);
 static int sftp_packet_ask(LIBSSH2_SFTP *sftp, unsigned char packet_type,
                            uint32_t request_id, unsigned char **data,