"Jump to Nav" links to primary links block if standard primary links disabled

The "Jump to Navigation" link is too important a feature for screen reader users to have break just because we want to use the Primary Links block instead of Zen's normal primary links display. This patch makes it so that, if the normal primary links display seems to be disabled, we try gamely to see if the primary links block will display somewhere, and if so, have "Jump to Navigation" link to that instead. There might be a more elegant way to do it, though…