Extend PATH to make xzcat available.

Extend PATH to make xzcat available.

Enforce python3 usage, as the newer version requires python3.

Empty values are considered not provided, so provide a template which
produces a file with a lone comment.

Instead of using openssl-1.0 and overriding the new versions to use
openssl, use openssl by default and override in the old versions to use
openssl-1.0

This reverts commit 852389ac.

This reverts commit bca3faca.

to avoid possible "no version information available" warnings.

See merge request nexedi/slapos!1453

  There are extra libraries and patches that are important to wendelin (bt5) usage,
  DataArray, DataSupply...

now we have a test extra

Some configure scripts, such as cmake's FindPython3, use explicit
python version such as python3.9 and can find system python, although
using python3 would be catched by the disabled python. With this change
we also catch attempt of using python3.9

We also print the executed command, it helps guessing what executed
python