Add a required command-line option to use pure-python crypto?

[skyguy]

To guard against human error, it might be a good idea to require a specific command-line option before enabling the pure-python crypto library. This would guard against the situation where the trusted crypto library has been accidentally uninstalled or removed from the PYTHONPATH, but the insecurity goes unnoticed as the RNS apps continue to function.

Maybe something like:
rnsd --use-purepython-crypto

A big, scary log message indicating the pure-python crypto library is active would also provide an extra safety net and reminder.
-Kevin