Tests: add usage note to check-rpc-mappings.py

Summary:

This test would previously fail without a user-friendly warning message,
if invoked with no arguments.

Backport of core PR13707
https://github.com/bitcoin/bitcoin/pull/13707/files

Depends on D3772.

Also updated the copyright so the file is in sync with core.

Test Plan:

./test/lint/check-rpc-mappings.py

Should return:

Usage: ./test/lint/check-rpc-mappings.py ROOT-DIR

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3779