LUDiagnoseOS: add more fields, cleanup

This patch exports all the way from backend a new field ‘api_version’
which holds the list of support API versions, and exposes the (already
computed) ‘parameters’ field.

The patch also reworks (again) the field calculation in its Exec()
method. All callers of LUDiagnoseOS pass in the 'valid' and 'variants'
parameters, thus having the special casing of whether to compute or not
the validity seems overkill. We move to a model where we always compute
these across-nodes arguments, in order to simplify the code, and we also
change the parameters set to be intersection of all node's values (which
means a change in description will drop the parameter from the list of
parameters).

Additionally, we update scripts/gnt-os, which was broken for multi-dir
OSes since the introduction of variants…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>