Add a Y/N question to the user and a force option for the --save-rc-stanza

Hi Michael,

I think its ready, and I have fixed mrash/fwknop#67.

The user is prompted when some important variables are going to be overwritten when the -save-rc-stanza argument is used through the command line.

A new force-stanza argument allows to bypass the above prompt and update the stanza with the values set on the command line or remove entries if they are not specified.

I have also added some tests to the test suite and fixed the manpage for the key arguments (rijndael and hmac).

Regards,