Allow setting TOR_CONTROL_PASSWD via about:config

All the other environment variables mentioned here https://gitlab.torproject.org/legacy/trac/-/wikis/TorBrowserBundleSAQ#using-an-external-tor-socks-daemon have an equivalent about:config pref. TOR_CONTROL_PASSWD does not. Can you please add a pref for TOR_CONTROL_PASSWD?