Tweak ssl(8)'s text about EC generation. Streamline by using "ecparam

Tweak ssl(8)'s text about EC generation. Streamline by using "ecparam
-genkey" rather than separately generating parameters and key. Give a
clue that some CAs accept only prime256v1. Show the user where to stop
if they're just generating a private key for acme-client and therefore
don't need to generate a csr or cert manually. Add xr to acme-client(1)
suggest by tb@.

ok jmc tb