Add -uaclientname and -uaclientversion config options to set the client name and version of the user agent in the version message.

Summary: Since BCHD unconditionally disconnects from nodes containing the "Bitcoin ABC" string, we cannot use it to connect to a Bitcoin ABC node, even if connected directly using -connect. This option allows to circumvent that feature.

Test Plan: ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: Mengerian, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9628