.gitignore: organization, comments, Vim, misc

* Made some more sections headed with a comment and moved lines there.

* Added a Vim modeline so that Vim will treat .gitingore as a config
  file.

* If the same file is opened mulitple times by separate instances of
  Vim, the first swap file will be .FOO.swp, the second .FOO.swo,
  the third .FOO.swn, and so on; deal with having up to three
  swap files.

* A "/" at the start of a filename matches the local repository root,
  so (for example) "/dat" will only match a file named "dat" if it's
  in the top-level directory, rather than matching "dat" anywhere.

* A "/" at the end of a filename means that the file must be a
  directory, so git-status will still note the file's presence if
  it's a link or normal file.

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