bench_ecmult: improve clarity of output
bench_ecmult: improve clarity of output

Previously "ecmult{,_multi} xg" meant multiplication with (x - 1) random points
and base point G. Now
- ecmult_{,multi_}xp means multiplication with x random points and
- ecmult_{,multi_}xp_g means multiplication with x random points and G

ElementsProject/secp256k1-zkp,peterdettman/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,bitcoin/secp256k1,bitcoin-core/secp256k1,bitcoin/secp256k1