Used args enumeration for both the update_rc() and add_rc_param().

Updated fwknop client to refer to the fwknop args enumeration rather
than the config variable names directly. This should make easier to
handle future changes of the variable name.

New function to validate a string matches a YES pattern in the configuration
file : is_yes_str().

The parse_rc_param() only returns at the end of the function, unless a fatal
error has been encountered.