Add share/rpcuser to dist. source code archive
As the legacy rpcuser and rpcpassword are deprected since 0.12.0, we should actually include the script to generate the new auth pair in the distributed source code archive.
Backport of core PR11530
Check the script is part of the generated archive
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D3554