rpcauth: Improve by using argparse and getpass modules

Summary:

This PR improves argument handling in rpcauth.py script by using
argparse module. Specifying - as password makes it prompt securely with
getpass module which prevents leaking passwords to bash history.

Backport of core PR14756
https://github.com/bitcoin/bitcoin/pull/14756/files

Depends on D3781.

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

Test Plan:

make check

./share/rpcauth/rpcauth.py foo --help

Check help is displayed correctly.

./share/rpcauth/rpcauth.py foo -

Check the script asks for password.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3782