V-C.O upgrade footnotes

[Poul-Henning Kamp]

I have upgraded the V-C.O server to FreeBSD 12.2-RELEASE and
it had some minor fallout:

The 5.1, 5.2, 6.0 and 6.1 builds all failed in the automatic run,
due to a new check in CLANG which spotted that a double cannot
represent a LONG_MAX precisely.

For 5.1 and 5.2, I patched a checked- out tree to build docs and
freeze them as tar files, as we have done with the 2.x, 3.x and
4.x docs which go on the webpage.

I did not commit the fixes to the 5.x branches, in order to not
give anybody false hope that we still maintain them.

For 6.0 and 6.1 I committed the fix to the branches so and
the docs continues to be built when commits to those branches
happen.

Poul-Henning