RF: smoother genRsaKeys; new test with 8192-bit key

- genRsaKeys is more user-friendly but still basic
- command line option --genrsa to start dialog
- use aes256 for passphrases (just to standardize on aes256)
- default to 4096 bits; 2048 is fine if good entropy
- remove most options; require passphrase, or generate one
- better clean-up if KeyboardInterrupt
- test 8192-bit dec(enc()) using known keys