json: Keep interpolation state in JSONParserContext

The recursive descent parser passes along a pointer to
JSONParserContext.  It additionally passes a pointer to interpolation
state (a va_alist *) as needed to reach its consumer
parse_interpolation().

Stuffing the latter pointer into JSONParserContext saves us the
trouble of passing it along, so do that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-56-armbru@redhat.com>