Move KDF handling to ECDH_compute_key()

Move KDF handling to ECDH_compute_key()

In OpenSSL e2285d87, the KDF handling was moved from the compute_key()
method into the public API. A consequence of this change is that the
ECDH_compute_key() API no longer returns -1 for some errors. Existing
checks for <= 0 are safe as are those checking for the exact length as
return value, which is all what the ecosystem seems to be doing.

ok jsing