Rename rpcuser.py to rpcauth.py

Summary:

This script creates `rpcauth` entries for bitcoin.conf,
not the deprecated `rpcuser` entry, so this changes the name
of the script to match.

Backport of core PR11836
https://github.com/bitcoin/bitcoin/pull/11836/files

Depends on D3553 and D3554

Test Plan:

./share/rpcauth/rpcauth.py foo

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3555