Improve handling of multiple footnote anchors

[windymilla]

Where several anchors refer to the same footnote, things aren't handled as well as they could be (in GG1 too, e.g. HTML generation doesn't generate links for the earlier anchors, only the last one with that number).

In an ideal world, all the anchors would appear in the message list in Footnote Fixup. Need to decide how to find them, and which footnote to link them to.

One possibility would be to search for anchors and then find the footnote for each anchor. ATM GG1 & 2 search for footnotes and then search back for the nearest same-numbered anchor. Since you can have many anchors for one footnote, but not many footnotes for one anchor, this alternative might be fruitful.

OTOH, there may be a good reason that GG1 was written the way it is.

[windymilla]

Just linking to #503