ECDSA: remove nonce padding (delegated to EC_POINT_mul)

* EC_POINT_mul is now responsible for constant time point multiplication
  (for single fixed or variable point multiplication, when the scalar is
  in the range [0,group_order), so we need to strip the nonce padding
  from ECDSA.
* Entry added to CHANGES
* Updated EC_POINT_mul documentation
  - Integrate existing EC_POINT_mul and EC_POINTs_mul entries in the
    manpage to reflect the shift in constant-time expectations when
    performing a single fixed or variable point multiplication;
  - Add documentation to ec_method_st to reflect the updated "contract"
    between callers and implementations of ec_method_st.mul.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6070)
