Allow generating key without reading config

[Dreamsorcerer]

It would be nice to be able to generate a simple key without worrying about config values in .fwknoprc. Currently I get:

> fwknop --key-gen --hmac-key-len=128
Must specify --gpg-recipient-key when GPG is used.

Because I have USE_GPG Y in the [default] section, as I want to use it on all servers. The key gen does not need to worry about these parameters.

[Dreamsorcerer]

I've just spotted that there is a --no-rc-file option. So, if it's non trivial to do this automatically on key gen, then using that argument will be good enough for me.