Features that begin with a '*' have some kind of global scope. Since
that, by itself, doesn't mean much, the second character of the
feature name must tell us what type of global feature it is. The two
currently supported are:

'&' - 'one is okay' - don't penalize the first pnode that has this
    feature, but penalize subsequent ones. Will be used to prefer
    not to give people multiple nodes at the sime widearea site.
'!' - 'more than one is okay' - penalize the first pnode that has
    this feature, but not subsequent ones. This could be used to let
    assign pick experiments to swap out in order to let the current
    experiment map. The idea being that once you've decided that one
    of the experiment's nodes is getting stolen, you might as well
    swap the whole thing.

For now, these global features should not have corresponding desires,
because it's not clear what the right thing to do would be. But, we may
decide to add these semantics someday.