ecc api cleanup; summary:

- hide the EC_KEY structure definition in ec_lcl.c + add
  some functions to use/access the EC_KEY fields
- change the way how method specific data (ecdsa/ecdh) is
  attached to a EC_KEY
- add ECDSA_sign_ex and ECDSA_do_sign_ex functions with
  additional parameters for pre-computed values
- rebuild libeay.num from 0.9.7