json: Support %% in JSON strings when interpolating

json: Support %% in JSON strings when interpolating

The previous commit makes JSON strings containing '%' awkward to
express in templates: you'd have to mask the '%' with an Unicode
escape \u0025.  No template currently contains such JSON strings.
Support the printf conversion specification %% in JSON strings as a
convenience anyway, because it's trivially easy to do.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <20180823164025[email protected]>