Here's a new version of the patch. I fixed the keymap thing, and looked
carefully at the code called from `gnus-summary-mode'.

`turn-on-mailing-list-mode' clearly belongs in
`gnus-summary-setup-buffer', so I moved it up there. The format spec and
mark position stuff doesn't vary depending on the group, but I have
dreams of a day when it will, so I moved that up, too.

I added more docs and comments making the separation of concerns clear:
`gnus-summary-setup-buffer' handles group-dependent setup,
`gnus-summary-mode' handles group-independent stuff. That's also the
motivation for moving the three `make-local-variables', which would
otherwise be a pointless change.

I hope this is acceptable (I'm cc'ing Lars in case he has an opinion).
I'll run it locally for a week or so, then push to master.

Eric