The Requests library honors the *_PROXY environment variables by default
(a dangerous and insecure policy in the opinion of the MMGen Project).
Thus if the user had one of these variables set and explicitly requested the
`requests` backend via the `--rpc-backend` option, JSON-RPC communications
would have gone over the configured proxy.
Only the `mmgen-xmrwallet` command uses `requests` as its default backend,
so only it was affected by this vulnerability in the default configuration,
i.e. without use of `--rpc-backend`.
This patch sets `trust_env=False` to disable the dangerous behavior.
