naming: privkey vs seckey

[real-or-random]

We have a bunch of secp256k1_ec_privkey_ functions but then we have secp256k1_ec_seckey_verify. I think we should create a privkey alias for this function.

More generally, we sometimes write "private key" and sometimes "secret key", and all the privkey functions expect a seckey argument. This should be made consistent too.

[kroggen]

What about using the privkey as default and let the seckey as the alias? So the function names are consistent.

My vote goes to have the arguments named privkey too. Decreasing the amount of different names makes the code easier to understand.

[jonasnick]

The thing is called seckey everywhere except the ec_privkey function names. So let's keep that as a default.

[real-or-random]

Superseded by #671