- changed return values and parameters of recv_token()

- changed return values and parameters of recv_token()
  - it doesn't allocate memory for token, but uses the passed buffer allocated
    by the caller
  - returns number of read bytes or negative int on errors

- added read_token() to read exact number of bytes

- added new calls recv_gss_token, and send_gss_token to handle gssapi token with
  a prefix indicating their length