change transport to use unified error codes

I added three new public error codes, and then modified the return
codes we use in the transport layer to use the generic error codes
so that there won't be any risk of internal confusions due to
different error code sets.

include/libssh2.h

@@ -356,6 +356,9 @@ typedef struct _LIBSSH2_POLLFD {
 #define LIBSSH2_ERROR_PUBLICKEY_PROTOCOL        -36
 #define LIBSSH2_ERROR_EAGAIN                    -37
 #define LIBSSH2_ERROR_BUFFER_TOO_SMALL          -38
+#define LIBSSH2_ERROR_BAD_USE                   -39
+#define LIBSSH2_ERROR_COMPRESS                  -40
+#define LIBSSH2_ERROR_OUT_OF_BOUNDARY           -41
 
 /* Session API */
 LIBSSH2_API LIBSSH2_SESSION *