TRIVIAL: Fix log file message for client name to match CLIENT_NAME

Summary:
It annoyed me that log files start as such:

2017-08-20 22:03:52 Bitcoin version v0.15.0.0-315c6029f

We are Bitcoin ABC. I looked at the code and it turns out the init message for the log file doesn't use CLIENT_NAME.

With this trivial patch log files will start as follows:

2017-08-20 22:03:52 Bitcoin ABC version v0.15.0.0-315c6029f

Test Plan: make and run bitcoind and see log file

Reviewers: freetrader, deadalnix, schancel, #bitcoin_abc, sickpig

Reviewed By: freetrader, deadalnix, #bitcoin_abc, sickpig

Differential Revision: https://reviews.bitcoinabc.org/D475