move default-guessing stuff into arg parsing, s/wiki/site/
    
    Meh, this didn't come out as much obviously cleaner as I hoped,
    but it still seems like an improvement. Concentrate most of
    the 'guessing defaults' stuff into the arg parsing logic and
    have sub-command functions always use arg.foo. Couple of ugly
    hacks needed to make it fly.
    
    Also, rename 'wiki' to 'site' in most places, I'm gonna try and
    get this consistent throughout relval and wikitcms eventually.