Better optional argument handling.

Some options can take an optional argument. However, in this case GNU getopt
requires that the optional argument is right next to the option without
whitespace inbetween. If there is whitespace, getopt will treat it as a
non-option argument, but tinc ignored those without a warning. Now tinc will
allow optional arguments with whitespace inbetween, and will give an error when
it encounters any other non-option arguments.