build: Update supported versions.

INSTALL.quagga.tex:
  Given the statement that it's viewed as a bug if quagga doesn't
  build on OS versions on the list, prune the list to the set for
  which there would be near-universal agreement that it's a bug.

  Clarify that the response to a system on the list not building might
  be dropping it from the list. (Time marches on, and these lists are
  not necessarily maintained.  As an example, the comment saying
  FreeBSD4 support was iffy is now 6 years old.)

  Delete old discussion of ancient texinfo.

  Delete discussion of NetBSD versions before 4 (as no longer relevant).

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