Extract the packet length from the old-format length header

Extract the packet length from the old-format length header

From RFC 4880 [1]:

  Scalar numbers are unsigned and are always stored in big-endian
  format.  Using n[k] to refer to the kth octet being interpreted, the
  value of a two-octet scalar is ((n[0] << 8) + n[1]).  The value of a
  four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
  n[3]).

The struct big-endian byte-order character is '>' [2].

[1]: http://tools.ietf.org/search/rfc4880#section-3.1
[2]: http://docs.python.org/3/library/struct.html#byte-order-size-and-alignment