Don't use anymore the family argument.

1) For every block, for every executable call the prover using the
version switch and add the prover to the configuration if the version
match one of the version_ok or version_old but none of the version_bad
2) We consider that an executable name which appears in a block, but
which version isn't a version_ok, version_old or version_bad has an
unknown version
3) For every executable which have an unknown version, we add the
prover using the first block that contains it.

So the order of the block is used only when the version of an
executable appears in none of the block.

A block with more than one exec fields is now the same thing than if
you split the block into blocks containing one field.

New message field that allows to print a message when a prover is
detected. If a message is not present, we print ", Ok." if the version
is good (version_good) and not old, and " (it is an old version)." if
the version is old (version_old).

The field command can be missing in a block, in that case the block
defines a version known to be buggy: no prover config is generated.