Re-commit turning makeKnownLinkObj(), makeBrokenLinkObj(), makeLinkObj() into link...

Re-commit turning makeKnownLinkObj(), makeBrokenLinkObj(), makeLinkObj() into link() wrappers.  This should mean that all link processing now goes through link().  Per Brion's advice, remove the BrokenLink hook and replace it with LinkBegin and LinkEnd hooks.  All parser tests pass except the usual suspects.