Fixed ask_overwrite(). Generated keys are now stored in fwknoprc.

 * ask_overwrite() : when the user inputs more than one char when prompted,
   a second call to the function does not take the second char anymore.
   We parse all of the chars until we reach an LF char and discard all of them
   except the first one.
   The overwrite is requested only when the user sets 'y', if there is anything
   else we asssume 'N'.

 * When -k is used on the command line along with the --save-rc-stanza, the
   generated keys are also written in the stanza in fwknoprc.